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Selenophene based benzodithiophene polymers as potential candidates for optoelectronic applications
Dyes and Pigments 149 (2018) 639–643
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Selenophene based benzodithiophene polymers as potential candidates for optoelectronic applications
T
Chinna Bathulaa,∗, Shubhangi Khadtareb, Kezia Burugac, Abhijit Kadamd, Nabeen K. Shresthaa, Yong-Young Noha a
Department of Energy and Materials Engineering, Dongguk University, Seoul 04620, Republic of Korea Department of Chemistry, Institute of Materials Design, Hanyang University, Seoul 04763, Republic of Korea c Department of Chemical Engineering, National Institute of Technology, Suratkal 575025, Karnataka, India d Department of Chemical and Biochemical Engineering, Gachon University, Seongnam City, Seoul 461-701, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Keywords: Selenophene Stilles reaction Conjugated polymers Optoelectronics
This work reports on the synthesis and characterization of two novel conjugated polymers consisting of selenophene substituted benzo[1,2-b:4,5-b′]dithiophene (SeBDT) donor, and dithienothiadiazole[3,4-c]pyridine (DTPyT)-P1 or thieno[3,4]pyrroledione(TPD)-P2 acceptors. The synthesized polymers are characterized for the significant photophysical prerequisites essential for organic electronics such as strong and broad optical absorption, thermal stability, and compatible highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. The polymers are thermally stable up to 280–370 °C, and the optical band gaps for P1, P2 calculated from their film absorption edges are found to be 1.53 and 1.84 eV, respectively. In addition, the electrochemical studies of P1, P2 reveal the HOMO and LUMO energy levels of −5.02,-5.04 eV, and −3.49, −3.20 eV, respectively, suggesting these materials to be potential candidates for the applications in organic electronics.
1. Introduction
closer intermolecular π−π stacking in view of the attractive forces between the donor and acceptor units. Benzo[1,2-b:4,5-b′]dithiophene(BDT) [10,11], naphthodithiophene (NDT) [12,13], dithienothiadiazole[3,4-c]pyridine (DTPyT) [14], and thienopyrroledione (TPD) [15] have been proven to be exceptional donor and acceptor units in some of the high performing materials. Recently, a series of D-A copolymers using BDT as the donor and TPD as the acceptor have been synthesized for PSCs applications [16]. These copolymers have a great potential as the large-band gap materials in tandem PSCs because they simultaneously demonstrate a good efficiency and a large open circuit voltage (Voc). Some of the TPD-based [17] solar cells have exhibited PCEs up to 8.5% when fabricated and tested under inert atmosphere. On the other hand, Selenophene substituted polymers are reported to show promising optoelectronic properties [18,19]. These observations inspired us to work on the selenophene substituted benzo[1,2-b:4,5-b′]dithiophene (SeBDT) based polymers. In this work, we report on the design, synthesis, and optoelectronic characterization of two novel donor–acceptor types of conjugated polymers, Viz. poly[4,8-bis(5-(2-ethylhexyl)selenophene-2-yl)benzo [1,2-b:4,5-b′]dithiophene–diyl-alt-[4,7-bis(4-(2-ethylhexyl)thiophen-2-
Remarkable progress has been made on bulk heterojunction organic photovoltaics owing to its several advantages such as lightness, flexibility, low cost, and their application in smart card, radio frequency identification and displays [1–3]. Conjugated polymer backbones containing alternating electron rich donor and electron-poor acceptor units have emerged as a common approach in the design of low band gap materials [4]. By careful consideration of the repeating donor and acceptor units, control over the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of these polymers can be achieved [5]. This facilitates the design of a variety of chromophores with optimal light absorption properties for the injection/extraction of charges, thereby enhancing the performance of organic field effect transistors (OFET), and polymer solar cells (PSCs) [6–8]. Donor and acceptor (D–A) concept is one of the most efficient approaches to strengthen the intermolecular interactions between the neighbouring molecules by enhancing their molecular orbital overlapping. Hence, development of the alternating D–A copolymers is a versatile way to and achieve an enhanced device performance by enhancing the carrier mobility [9], because these polymers can exhibit
∗
Corresponding author. E-mail address: [email protected] (C. Bathula).
https://doi.org/10.1016/j.dyepig.2017.11.026 Received 14 September 2017; Received in revised form 1 November 2017; Accepted 13 November 2017 0143-7208/ © 2017 Elsevier Ltd. All rights reserved.
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Scheme 1. Synthesis of 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)selenophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene.
Scheme 2. Synthetic route for polymers.
yl)-[1,2,5]thiadiazolo[3,4-c]pyridine](P1), and poly[4,8-bis(5-(2ethylhexyl)selenophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene-diyl-alt[4,7-bis-5-(heptadecan-9-yl)-5H-thieno[3,4-c]pyrrole-4,6-dione(P2). Desired polymers were synthesized through Stille polymerization reaction by utilizing selenophene substituted benzo[1,2-b:4,5-b′]dithiophene (SeBDT) donor and dithienothiadiazole[3,4-c]pyridine (DTPyT), and thieno[3,4]pyrroledione (TPD) as acceptors. The important photoTable 1 Physical properties of the polymers. Polymer
Mna [kg/mol]
Mwa [kg/mol]
PDI
Yield [%]
Tdb [oC]
P1 P2
26.2 23.5
63.4 49.1
2.40 2.08
83 76
370 280
a The molecular weights were determined by using gel permeation chromatography (GPC) against polystyrene standards in chloroform as eluent. b Temperature resulting in 5% weight loss based on initial weight.
Fig. 1. TGA plots for the polymer, obtained with a heating rate of 10 °C min−1 under an inert atmosphere.
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Fig. 3. Cyclic voltammograms of polymers in 0.1 M Bu4NPF6/CH3CN, scan rate 50 mV s−1, Pt working electrodes.
2. Experimental
available solvents and reagents were used without further purification. The monomers 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)selenophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene [20], 4,7-bis(5-bromo-4-(2ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine [21], and 1,3-dibromo-5-(heptadecan-9-yl)-5H-thieno[3,4-c]pyrrole-4,6-dione [22] were prepared by previously described methods. Thin layer chromatography (TLC) was performed on silica gel plates precoated with a fluorescent indicator. Standardized silica gel was used for column chromatography purifications. Mixtures of eluent are given in v/v ratio. The synthesized compounds were characterized with 1H NMR and 13C NMR spectroscopy using a Bruker DPX-300 NMR spectrometer. Commercial NMR solvents obtained from Aldrich with TMS as internal standard were used, and chemical shifts are measured in δ ppm scale. UV–Vis spectroscopy were recorded using Lambda 20 (Perkin Elmer) diode array spectrophotometer at room temperature unless otherwise noted. All solutions in UV–Vis experiments were prepared in CHCl3. For the study of optical properties of the polymers, the thin films were prepared on quartz plates. Prior to deposition of polymer, plates were thoroughly cleaned with deionized water, chloroform and acetone followed by drying in an oven. The films were prepared by spin-coating polymer solutions (6 mg mL−1 in CHCl3) onto quartz substrates followed by drying the films at 40 °C. Cyclic voltammetry (CV) was run using Weis-500 work-station to investigate the electrochemical behaviour of the conjugated polymers, and to estimate their HOMO and LUMO energy levels. For the electrochemical properties of polymers, thin films were prepared on ITO, which is used as working electrode, platinum as counter electrode and Ag/AgCl as reference electrode, respectively. 0.1 M Bu4NPF6 was used as electrolyte in acetonitrile solution with scan rate of 50 mV s−1. The onset oxidation and reduction potentials of the polymers were estimated from the voltammograms, which correspond to the HOMO and LUMO energy levels, respectively. The number and weight average molecular weights of the polymers were determined by gel permeation chromatography (GPC; Viscotek) equipped with a TDA 302 detector and a PL-gel (Varian) column, using chloroform as the eluent and polystyrene as the standard. Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere at a heating rate of 10 °C min−1 with a Dupont 9900 analyzer. Elemental analyses were performed on a Vario III elemental analyzer.
2.1. Materials and methods
2.2. Synthesis
Selenophene, n-butyllithium, 2-ethylhexylbromide, SnCl2·2H2O, tetrakis(triphenylphosphine)palladium, trimethyltinchloride, diethylether, dimethylformamide(DMF), tetrahydrofuran (THF), toluene (99.8%, anhydrous), methanol, acetone, hexanes, and chloroform were purchased from Aldrich. Unless otherwise stated, all commercially
2.2.1. Synthesis of 2-(2-ethylhexyl)selenophene (2) To a stirred solution of selenophene (1) (5.0 g, 38 mmol) in anhydrous THF (100 mL), 2.5 M n-butyl lithium (16.8 mL, 41.9 mmol) was added slowly at −78 °C. Stirring was continued for 50 min, 7.5 mL of 2ethylhexylbromide (41.9 mmol) was added to the solution. The reaction
Fig. 2. UV–Vis absorption spectra of the polymers in chloroform solution and thin film state.
Table 2 Optical and electrochemical properties of the polymers. Polymer
λmaxa (Sol) (nm)
λmaxa (Film) (nm)
HOMOb (eV)
LUMOc (eV)
Egopt,d (eV)
440, 660
450, 700
−5.02
−3.49
1.53
557, 614
571, 628
−5.04
−3.20
1.84
P1 P2 a
The UV–Vis absorption spectra of the polymers were measured in chloroform solution and thin film. b HOMO levels of the polymer were determined from onset voltage of the first oxidation potential with reference to ferrocene at −4.8 eV. c LUMO levels of the polymer were estimated from the optical band gaps and the HOMO energy levels. d Optical band gap was calculated from the UV–Vis absorption onset in film.
physical prerequisites for application in organic electronics such as optical absorption, thermal stability, and HOMO-LUMO levels were determined for the synthesized molecules. The optoelectronic characterization of the synthesized polymers suggests these materials as potential candidates for the application in organic electronics.
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mixture was warmed to room temperature, kept stirring for another 16 h, and slowly poured into water. The mixture was extracted with ether. The organic layer was dried over MgSO4, filtrated, and evaporated. The crude product was purified by column chromatography on silica gel using petroleum ether as the eluent to give pure product (2) as colorless oil (3.8 g in 40% yield). 1H NMR (CDCl3, 300 MHz), δ (ppm): 7.79 (d, 1H), 7.12 (d-d, 1H), 6.92 (d, 1H), 2.82(d, 2H), 1.29–1.45 (m, 9H) 0.86–0.90 (t, 6H), 13C NMR (CDCl3, 100 MHz), δ (ppm): 145.19, 139.06, 137.51, 136.44, 127.61, 127.30, 125.30, 124.03, 123.29, 41.55, 34.33, 32.53, 29.42, 25.83, 23.06, 15.67, 11.00.
acetone, hexanes, and chloroform. The polymer was recovered as solid from the chloroform fraction by precipitation from methanol. The solid was dried under vacuum for 24 h at 40 °C. The yield and elemental analytical results of the polymer are as follows. Yield (296 mg, 83%). Mn = 26.2 kg/Mol, Mw = 63.4 kg/mol, PDI (Mw/Mn) = 2.4.1H NMR (CDCl3, 300 MHz), δ (ppm): 8.60 (br, 1H), 8.32(br, 1H), 7.6 (br, 1H), 7.45 (br, 2H), 7.25 (br, 2H), 6.78 (br, 2H), 2.82 (br, 4H), 2.48 (br, 4H), 1.70 (br, 2H), 1.60 (br, 2H), 1.40–1.50 (br, 16H), 1.28–1.1 (br, 16 H), 0.91 (br, 12H), 0.80 (br, 12H), Elemental analysis calculated, C, 63.75; H, 6.83; N, 3.43; S, 13.09; found C, 63.72; H, 6.763; N, 3.40; S, 13.02.
2.2.2. Synthesis of 4,8-bis(5-(2-ethylhexyl)selenophene-2-yl)benzo[1,2-b; 4,5-b′]dithiophene (4) Using a 250 mL argon purged flask, n-butyl lithium (2.5 M, 5.4 mL, 1.8 mmol) was added dropwise to a solution of 2-(2-ethylhexyl) selenophene (2) (3.3 g, 1.8 mmol) in THF (50 mL) at 0 °C. The mixture was then warmed to 50 °C and stirred for 1 h. Subsequently, 4,8-dehydrobenzo[l,2-b:4,5- b′]dithiophene-4,8-dione (3) (1 g, 4.5 mmol) was added to the reaction mixture, which was then stirred for 1 h at 50 °C. After cooling the reaction mixture to ambient temperature, a mixture of SnCl2·2H2O (8.2 g, 3.6 mmol) in 10% HCl (15 mL) was added and the mixture was stirred for additional 1.5 h, after which it was poured into ice water. The mixture was extracted with diethyl ether twice and the combined organic phases were concentrated to obtain the crude product. Further purification was carried out by column chromatography on silica gel using petroleum ether as the eluent to obtain pure compound (4) as pale yellow viscous liquid solidifies on cooling (1.5 g, yield 50%). 1H NMR (CDCl3, 300 MHz), δ (ppm): 7.67(d, 2H), 7.47 (d, 2H), 7.42 (d, 2H), 7.02 (d, 2H), 2.94 (d, 4H), 1.68 (m, 2H), 1.49–1.35 (br, 16H), 1.0–0.92 ppm (m, 12H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 145.19, 139.06, 137.51, 136.44, 127.61, 127.30, 125.30, 124.03, 123.29, 41.55, 34.33, 32.53, 29.42, 25.83, 23.06, 15.67, 11.00.
2.2.5. Poly[4,8-bis(5-(2-ethylhexyl)selenophene-2-yl)benzo[1,2-b:4,5-b′] dithiophene-diyl -alt-[4,7-(5-(heptadecan-9-yl)-5H-thieno[3,4-c]pyrrole4,6-dione) (P2) Monomer 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)selenophen-2yl)benzo[1,2-b;4,5-b′] dithiophene (5) (300 mg, 0.3 mmol) and 1,3-dibromo-5-(heptadecan-9-yl)-5H-thieno[3,4-c]pyrrole-4, 6-dione (165 mg, 0.3 mmol) (7) were mixed in 4 mL of toluene and 1 mL of DMF to the pressure tube. After being purged with argon for 5 min, Pd(PPh3)4 (15 mg) was added as the catalyst, and the mixture was then purged with argon for 25 min. The reaction mixture was stirred and heated at 110 °C for 24 h. Then the reaction mixture was cooled to room temperature, and the polymer was precipitated by addition of 50 mL methanol, filtered and dried. Polymer was redissolved in chlorobenzene and precipitated in 50 mL methanol. The precipitate was then filtered and subjected to Soxhlet extraction with methanol, acetone, hexanes, and chloroform. The polymer was recovered as solid from the chloroform fraction by precipitation from methanol. The solid was dried under vacuum for 24 h at 40 °C. The yield and elemental analytical results of the polymer are as follows. Yield (240 mg, 76%). Mn = 23.5 kg/Mol, Mw = 49.1 kg/mol, PDI (Mw/Mn) = 2.08.1H NMR (CDCl3, 300 MHz), δ (ppm): 7.50 (br, 2H), 7.42 (br, 2H), 6.80 (br, 2H), 3.4 (br, 1H), 2.86 (br, 4H), 1.80–1.75 (br, 2H), 1.7 (2H),1.60–1.58 (br, 2H), 1.28–1.1 (br, 40 H), 0.90 (br, 12H), 0.8 (br, 6H). Elemental analysis calculated, C, 64.81; H, 6.77; N, 1.33; S, 9.11; found C, 64.76; H, 6.68; N, 1.27; S, 9.02.
2.2.3. 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)selenophene-2-yl) benzo[1,2-b:4,5-b′] dithiophene (5) A solution of compound (4) (1.5 g, 2.2 mmol) in THF (40 mL) at 0 °C was placed in a 100 mL argon purged flask, and then n-butyl lithium (1.6 M, 4.1 mL, 6.6 mmol) was added. The reaction mixture was then stirred for 2 h at ambient temperature. Subsequently, chlorotrimethylstannane (1.0 M in hexane, 8.9 mL, 8.8 mmol) was added and the mixture was stirred for an additional 1 h at ambient temperature. Then, the mixture was poured into ice water. Extracted by diethyl ether and the combined organic phase were concentrated to obtain crude compound. Further purification was carried out by recrystallization using ethanol to obtain the pure compound (5) as a light-yellow solid (1.4 g, yield 63%).1H NMR (CDCl3, 300 MHz), δ (ppm): 7.52 (s, 2H), 7.24 (d, 2H), 6.88 (d, 2H), 2.75 (d, 4H), 1.54 (m, 2H), 1.28–1.1 (br, 16 H), 0.80 (m, 12H), 0.21 (s, 18H). 13C NMR (CDCl3, 100 MHz), δ (ppm): 145.39, 143.28, 142.24, 138.00, 137.31, 131.18, 127.54, 125.30, 122.41, 41.57, 34.25, 32.52, 28.97, 25.81, 23.04, 15.42, 10.99, −8.34.
3. Results and discussion Conjugated polymers P1 and P2 are synthesized by Pd(PPh3)4 catalyzed Stille copolymerization reaction using the monomers, 2,6-Bis (trimethyltin)-4,8-bis(5-(2-ethylhexyl)selenophene-2-yl)benzo [1,2b:4,5-b′]dithiophene (SeBDT) as a donor, 4,7-bis(5-bromo-4-(2-ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine (DTPyT) or dibromo-5-(heptadecan-9-yl)-5H-thieno[3,4-c]pyrrole-4,6-dione (TPD) as acceptors, respectively. Synthetic route for donor monomer SeBDT is shown in Scheme 1, and the copolymerization is illustrated in Scheme 2. Ethylhexyl alkyl side chain group is introduced on selenophene substituted benzodithiophene as well as on dithienothiadiazole[3,4-c] pyridine and octylnonyl chain is incorporated on thienopyrroledione monomers to ensure the solubility of the corresponding polymers. After the completion of polymerization reaction, the crude polymers were collected by precipitation in methanol and filtration. Further purification is carried by Soxhlet apparatus, using methanol, acetone, hexane and chloroform successively to remove the by-products. The final polymers were obtained by evaporation of chloroform and precipitating in methanol. Polymers are filtered and dried in vacuum. Thus, the obtained polymers P1 and P2 are readily soluble in common organic solvents such as chloroform, chlorobenzene and dichlorobenzene, indicating easy processability during fabrication of organic electronic devices. The number average molecular weights (Mn) and dispersity (PDIs) of the copolymers were determined by the gel permeation chromatography (GPC) analysis with a polystyrene standard calibration in chloroform eluent, and were in the range of 23.5–26.2 kg mol−1 with a dispersity of 2.08–2.40 (Table 1). The structures of all the products were confirmed by 1H NMR spectroscopy. In addition, the polymers were characterized by their elemental analysis.
2.2.4. Poly[4,8-bis(5-(2-ethylhexyl)selenophene-2-yl)benzo[1,2-b:4,5-b′] dithiophene–diyl-alt-[4,7-bis(4-(2-ethylhexyl)thiophen-2-yl)-[1,2,5] thiadiazolo[3,4-c]pyridine](P1) Monomer 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)selenophen2-yl)benzo[1,2-b; 4,5-b′]dithiophene (5) (300 mg, 0.3 mmol) and 4,7bis(5-bromo-4-(2-ethylhexyl)thiophen-2-yl)-[1,2,5] thiadiazolo [3,4-c] pyridine (6) (204 mg, 0.3 mmol) were mixed in 4 mL of toluene and 1 mL of DMF to the pressure tube. After being purged with argon for 5 min, Pd(PPh3)4 (15 mg) was added as the catalyst, and the mixture was then purged with argon for 25 min. The reaction mixture was stirred and heated at 110 °C for 24 h. Then the reaction mixture was cooled to room temperature, and the polymer was precipitated by addition of 50 mL methanol, filtered and dried. Polymer was redissolved in chlorobenzene and precipitated in 50 mL methanol. The precipitate was then filtered and subjected to Soxhlet extraction with methanol, 642
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Thermogravimetric analysis (TGA) of polymers P1 and P2 was investigated to determine the thermal stability. Fig. 1 shows the TGA chart, where the onset temperature with 5% weight loss of P1 is above 370 °C and for P2, it is around 280 °C (Table 1). This impressive thermal stability is comparable to alkyl substituted BDT polymers, which are recommended for organic solar cell and organic field effect transistor applications. Interestingly, polymer P1 containing DTPyT exhibited high thermal stability due to the presence of dithienothiadiazole[3,4-c] pyridine moiety as acceptor part in the polymer backbone as compared to the polymer P2 containing TPD moiety as an acceptor. A similar thermal stability trend is observed for alkyl substituted BDT based polymers [12,14]. The thermal stability of these polymers is adequate for their applications in optoelectronic devices. The absorption spectra of the polymers in chloroform solution and in the thin films form are shown in Fig. 2. Optical data including the maximum absorption peak wavelengths (λmax) and the optical band gap (Egopt) are summarized in Table 2. The entire absorption spectra feature broad absorption bands from 350 nm to 900 nm long wavelength regions. The absorbance bands in chloroform solution for P1 were observed at 440 nm and 660 nm, while those of P2 were found at 557 nm and 614 nm. The higher energy absorbance are attributed to localized π-π* transitions while the lower energy bands were associated with an intramolecular charge transfer (ICT) between the donor and acceptor similar to those characterized by Jespersen et al. [23,24]. The polymer films were slightly red-shifted by 14–40 nm, which is most likely due to the high coplanarity or enhanced intermolecular electronic interactions in the solid state, leading to stability of a lower energy excited state. The optical band gaps (Egopt) for P1 and P2 determined from their film absorption edges (λonset) are about 1.53 and 1.84 eV, respectively. As shown in Fig. 3, the onset oxidation potentials (Eox) of P1 and P2 are 0.47 and 0.49 V, respectively versus Ag/AgCl reference electrode. Using ferrocene reference value of −4.8 eV below the vacuum level [24], the HOMO energy levels of the polymers were calculated as EHOMO = −(Eox − EFc + 4.8) (eV), wherein EFc = 0.25 eV is the potential of the internal standard of the ferrocene/ferrocenium ion (Fc/Fc+) couple. Thus, the HOMO levels were determined to be −5.02 eV for P1 and −5.04 eV for P2. Based on the optical band gap (Egopt) values and HOMO energy levels, the LUMO energy levels of P1 and P2 were estimated to be −3.49 and −3.20 eV, respectively, The results are summarized in Table 2. Compared to P1 (HOMO = −5.02 eV), P2 shows a deeper HOMO energy level of −5.04 eV, which indicates the presence of strong π-π stacking among the polymer backbone and formation of ordered arrangements in their solid films. In comparison, both polymers show almost similar optical properties. In contrast, in the case of polymer films, P2 shows a clear red shift than does P1. Note that the thermal, optical and electrochemical properties of selenophene substituted benzo[1,2-b:4,5-b′]dithiophene (SeBDT) polymers are comparable to that of alkyl substituted polymers (Supporting Information Table S1 and Table S2) employed in organic electronics. This suggests the polymers P1 and P2 as potential candidates for organic electronic applications.
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Electronic structure of small band gap oligomers based on cyclopentadithiophenes and acceptor units. J Mat. Chem 2009;19:5343–50. [19] Chen H-Y, Hou JH, Zhang SQ, Liang YY, Yang GW, Yang Y, et al. Polymer solar cells with enhanced open-circuit voltage and efficiency. Nat Photonics 2009;3:649–53. [20] Byun YS, Kim JH, Park JB, Kang I-N, Jin SJ, Hwang D-H. Full donor-type conjugated polymers consisting of alkoxy- or alkylselenophene-substituted benzodithiophene and thiophene units for organic photovoltaic devices. Synth Met 2013;168:23–30. [21] Bathula C, Lee SK, Kalode P, Sachin B, Belavagi NS, Khazi IM, Kang Y. Synthesis and photophysical studies of thiadiazole[3,4-c]pyridine copolymer based organic fieldeffect transistors. J Fluoresc 2016;26:1045–52. [22] Yuan J, Zhai Z, Dong H, Li J, Jiang Z, Li Y, et al. Efficient polymer solar cells with a high Open circuit voltage of 1 volt. Adv Funct Mater 2016;23:885–92. [23] Jespersen KG, Beenken WJ, Zaushitsyn Y, Yartsev A, Andersson M, Pullerits T, et al. The electronic states of polyfluorene copolymers with alternating donor-acceptor units. J Chem Phys 2004;121:12613–7. [24] Yang H-Y, Sun Y-K, Chuang Y-Y, Wang T-L, Shieh Y-T, Chen W-J. Electrochemical impedance parameters elucidate performance of carbazole–triphenylamine–ethylenedioxythiophene-based molecules in dye-sensitized solar cells. Electrochimica Acta 2012;2012(69):256–67.
4. Conclusion In summary, we report the synthesis and characterization of two novel selenophene substituted benzo[1,2-b:4,5-b′]dithiophene (SeBDT) containing conjugated polymers (P1 and P2) consisting of dithienothiadiazole [3,4-c]pyridine (DTPyT) and thieno[3,4]pyrroledione (TPD) acceptors synthesized through Stille polymerization reaction. Polymers exhibited good thermal stability and broad absorption bands. The HOMO and LUMO energy levels of the polymer P1 and P2 were found to be −5.02, −5.04 eV and −3.49, −3.20 eV respectively. Synthesized conjugated polymers are highly soluble in common organic solvents, which is a critical factor for fabricating electronic device. The photophysical properties and optoelectronic studies of the synthesized polymers suggest these materials to be the promising candidates for the application in organic 643
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Selenophene based benzodithiophene polymers as potential candidates for optoelectronic applications
T
Chinna Bathulaa,∗, Shubhangi Khadtareb, Kezia Burugac, Abhijit Kadamd, Nabeen K. Shresthaa, Yong-Young Noha a
Department of Energy and Materials Engineering, Dongguk University, Seoul 04620, Republic of Korea Department of Chemistry, Institute of Materials Design, Hanyang University, Seoul 04763, Republic of Korea c Department of Chemical Engineering, National Institute of Technology, Suratkal 575025, Karnataka, India d Department of Chemical and Biochemical Engineering, Gachon University, Seongnam City, Seoul 461-701, Republic of Korea b
A R T I C L E I N F O
A B S T R A C T
Keywords: Selenophene Stilles reaction Conjugated polymers Optoelectronics
This work reports on the synthesis and characterization of two novel conjugated polymers consisting of selenophene substituted benzo[1,2-b:4,5-b′]dithiophene (SeBDT) donor, and dithienothiadiazole[3,4-c]pyridine (DTPyT)-P1 or thieno[3,4]pyrroledione(TPD)-P2 acceptors. The synthesized polymers are characterized for the significant photophysical prerequisites essential for organic electronics such as strong and broad optical absorption, thermal stability, and compatible highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. The polymers are thermally stable up to 280–370 °C, and the optical band gaps for P1, P2 calculated from their film absorption edges are found to be 1.53 and 1.84 eV, respectively. In addition, the electrochemical studies of P1, P2 reveal the HOMO and LUMO energy levels of −5.02,-5.04 eV, and −3.49, −3.20 eV, respectively, suggesting these materials to be potential candidates for the applications in organic electronics.
1. Introduction
closer intermolecular π−π stacking in view of the attractive forces between the donor and acceptor units. Benzo[1,2-b:4,5-b′]dithiophene(BDT) [10,11], naphthodithiophene (NDT) [12,13], dithienothiadiazole[3,4-c]pyridine (DTPyT) [14], and thienopyrroledione (TPD) [15] have been proven to be exceptional donor and acceptor units in some of the high performing materials. Recently, a series of D-A copolymers using BDT as the donor and TPD as the acceptor have been synthesized for PSCs applications [16]. These copolymers have a great potential as the large-band gap materials in tandem PSCs because they simultaneously demonstrate a good efficiency and a large open circuit voltage (Voc). Some of the TPD-based [17] solar cells have exhibited PCEs up to 8.5% when fabricated and tested under inert atmosphere. On the other hand, Selenophene substituted polymers are reported to show promising optoelectronic properties [18,19]. These observations inspired us to work on the selenophene substituted benzo[1,2-b:4,5-b′]dithiophene (SeBDT) based polymers. In this work, we report on the design, synthesis, and optoelectronic characterization of two novel donor–acceptor types of conjugated polymers, Viz. poly[4,8-bis(5-(2-ethylhexyl)selenophene-2-yl)benzo [1,2-b:4,5-b′]dithiophene–diyl-alt-[4,7-bis(4-(2-ethylhexyl)thiophen-2-
Remarkable progress has been made on bulk heterojunction organic photovoltaics owing to its several advantages such as lightness, flexibility, low cost, and their application in smart card, radio frequency identification and displays [1–3]. Conjugated polymer backbones containing alternating electron rich donor and electron-poor acceptor units have emerged as a common approach in the design of low band gap materials [4]. By careful consideration of the repeating donor and acceptor units, control over the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of these polymers can be achieved [5]. This facilitates the design of a variety of chromophores with optimal light absorption properties for the injection/extraction of charges, thereby enhancing the performance of organic field effect transistors (OFET), and polymer solar cells (PSCs) [6–8]. Donor and acceptor (D–A) concept is one of the most efficient approaches to strengthen the intermolecular interactions between the neighbouring molecules by enhancing their molecular orbital overlapping. Hence, development of the alternating D–A copolymers is a versatile way to and achieve an enhanced device performance by enhancing the carrier mobility [9], because these polymers can exhibit
∗
Corresponding author. E-mail address: [email protected] (C. Bathula).
https://doi.org/10.1016/j.dyepig.2017.11.026 Received 14 September 2017; Received in revised form 1 November 2017; Accepted 13 November 2017 0143-7208/ © 2017 Elsevier Ltd. All rights reserved.
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Scheme 1. Synthesis of 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)selenophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene.
Scheme 2. Synthetic route for polymers.
yl)-[1,2,5]thiadiazolo[3,4-c]pyridine](P1), and poly[4,8-bis(5-(2ethylhexyl)selenophene-2-yl)benzo[1,2-b:4,5-b′]dithiophene-diyl-alt[4,7-bis-5-(heptadecan-9-yl)-5H-thieno[3,4-c]pyrrole-4,6-dione(P2). Desired polymers were synthesized through Stille polymerization reaction by utilizing selenophene substituted benzo[1,2-b:4,5-b′]dithiophene (SeBDT) donor and dithienothiadiazole[3,4-c]pyridine (DTPyT), and thieno[3,4]pyrroledione (TPD) as acceptors. The important photoTable 1 Physical properties of the polymers. Polymer
Mna [kg/mol]
Mwa [kg/mol]
PDI
Yield [%]
Tdb [oC]
P1 P2
26.2 23.5
63.4 49.1
2.40 2.08
83 76
370 280
a The molecular weights were determined by using gel permeation chromatography (GPC) against polystyrene standards in chloroform as eluent. b Temperature resulting in 5% weight loss based on initial weight.
Fig. 1. TGA plots for the polymer, obtained with a heating rate of 10 °C min−1 under an inert atmosphere.
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Fig. 3. Cyclic voltammograms of polymers in 0.1 M Bu4NPF6/CH3CN, scan rate 50 mV s−1, Pt working electrodes.
2. Experimental
available solvents and reagents were used without further purification. The monomers 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)selenophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene [20], 4,7-bis(5-bromo-4-(2ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine [21], and 1,3-dibromo-5-(heptadecan-9-yl)-5H-thieno[3,4-c]pyrrole-4,6-dione [22] were prepared by previously described methods. Thin layer chromatography (TLC) was performed on silica gel plates precoated with a fluorescent indicator. Standardized silica gel was used for column chromatography purifications. Mixtures of eluent are given in v/v ratio. The synthesized compounds were characterized with 1H NMR and 13C NMR spectroscopy using a Bruker DPX-300 NMR spectrometer. Commercial NMR solvents obtained from Aldrich with TMS as internal standard were used, and chemical shifts are measured in δ ppm scale. UV–Vis spectroscopy were recorded using Lambda 20 (Perkin Elmer) diode array spectrophotometer at room temperature unless otherwise noted. All solutions in UV–Vis experiments were prepared in CHCl3. For the study of optical properties of the polymers, the thin films were prepared on quartz plates. Prior to deposition of polymer, plates were thoroughly cleaned with deionized water, chloroform and acetone followed by drying in an oven. The films were prepared by spin-coating polymer solutions (6 mg mL−1 in CHCl3) onto quartz substrates followed by drying the films at 40 °C. Cyclic voltammetry (CV) was run using Weis-500 work-station to investigate the electrochemical behaviour of the conjugated polymers, and to estimate their HOMO and LUMO energy levels. For the electrochemical properties of polymers, thin films were prepared on ITO, which is used as working electrode, platinum as counter electrode and Ag/AgCl as reference electrode, respectively. 0.1 M Bu4NPF6 was used as electrolyte in acetonitrile solution with scan rate of 50 mV s−1. The onset oxidation and reduction potentials of the polymers were estimated from the voltammograms, which correspond to the HOMO and LUMO energy levels, respectively. The number and weight average molecular weights of the polymers were determined by gel permeation chromatography (GPC; Viscotek) equipped with a TDA 302 detector and a PL-gel (Varian) column, using chloroform as the eluent and polystyrene as the standard. Thermogravimetric analysis (TGA) was performed under a nitrogen atmosphere at a heating rate of 10 °C min−1 with a Dupont 9900 analyzer. Elemental analyses were performed on a Vario III elemental analyzer.
2.1. Materials and methods
2.2. Synthesis
Selenophene, n-butyllithium, 2-ethylhexylbromide, SnCl2·2H2O, tetrakis(triphenylphosphine)palladium, trimethyltinchloride, diethylether, dimethylformamide(DMF), tetrahydrofuran (THF), toluene (99.8%, anhydrous), methanol, acetone, hexanes, and chloroform were purchased from Aldrich. Unless otherwise stated, all commercially
2.2.1. Synthesis of 2-(2-ethylhexyl)selenophene (2) To a stirred solution of selenophene (1) (5.0 g, 38 mmol) in anhydrous THF (100 mL), 2.5 M n-butyl lithium (16.8 mL, 41.9 mmol) was added slowly at −78 °C. Stirring was continued for 50 min, 7.5 mL of 2ethylhexylbromide (41.9 mmol) was added to the solution. The reaction
Fig. 2. UV–Vis absorption spectra of the polymers in chloroform solution and thin film state.
Table 2 Optical and electrochemical properties of the polymers. Polymer
λmaxa (Sol) (nm)
λmaxa (Film) (nm)
HOMOb (eV)
LUMOc (eV)
Egopt,d (eV)
440, 660
450, 700
−5.02
−3.49
1.53
557, 614
571, 628
−5.04
−3.20
1.84
P1 P2 a
The UV–Vis absorption spectra of the polymers were measured in chloroform solution and thin film. b HOMO levels of the polymer were determined from onset voltage of the first oxidation potential with reference to ferrocene at −4.8 eV. c LUMO levels of the polymer were estimated from the optical band gaps and the HOMO energy levels. d Optical band gap was calculated from the UV–Vis absorption onset in film.
physical prerequisites for application in organic electronics such as optical absorption, thermal stability, and HOMO-LUMO levels were determined for the synthesized molecules. The optoelectronic characterization of the synthesized polymers suggests these materials as potential candidates for the application in organic electronics.
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mixture was warmed to room temperature, kept stirring for another 16 h, and slowly poured into water. The mixture was extracted with ether. The organic layer was dried over MgSO4, filtrated, and evaporated. The crude product was purified by column chromatography on silica gel using petroleum ether as the eluent to give pure product (2) as colorless oil (3.8 g in 40% yield). 1H NMR (CDCl3, 300 MHz), δ (ppm): 7.79 (d, 1H), 7.12 (d-d, 1H), 6.92 (d, 1H), 2.82(d, 2H), 1.29–1.45 (m, 9H) 0.86–0.90 (t, 6H), 13C NMR (CDCl3, 100 MHz), δ (ppm): 145.19, 139.06, 137.51, 136.44, 127.61, 127.30, 125.30, 124.03, 123.29, 41.55, 34.33, 32.53, 29.42, 25.83, 23.06, 15.67, 11.00.
acetone, hexanes, and chloroform. The polymer was recovered as solid from the chloroform fraction by precipitation from methanol. The solid was dried under vacuum for 24 h at 40 °C. The yield and elemental analytical results of the polymer are as follows. Yield (296 mg, 83%). Mn = 26.2 kg/Mol, Mw = 63.4 kg/mol, PDI (Mw/Mn) = 2.4.1H NMR (CDCl3, 300 MHz), δ (ppm): 8.60 (br, 1H), 8.32(br, 1H), 7.6 (br, 1H), 7.45 (br, 2H), 7.25 (br, 2H), 6.78 (br, 2H), 2.82 (br, 4H), 2.48 (br, 4H), 1.70 (br, 2H), 1.60 (br, 2H), 1.40–1.50 (br, 16H), 1.28–1.1 (br, 16 H), 0.91 (br, 12H), 0.80 (br, 12H), Elemental analysis calculated, C, 63.75; H, 6.83; N, 3.43; S, 13.09; found C, 63.72; H, 6.763; N, 3.40; S, 13.02.
2.2.2. Synthesis of 4,8-bis(5-(2-ethylhexyl)selenophene-2-yl)benzo[1,2-b; 4,5-b′]dithiophene (4) Using a 250 mL argon purged flask, n-butyl lithium (2.5 M, 5.4 mL, 1.8 mmol) was added dropwise to a solution of 2-(2-ethylhexyl) selenophene (2) (3.3 g, 1.8 mmol) in THF (50 mL) at 0 °C. The mixture was then warmed to 50 °C and stirred for 1 h. Subsequently, 4,8-dehydrobenzo[l,2-b:4,5- b′]dithiophene-4,8-dione (3) (1 g, 4.5 mmol) was added to the reaction mixture, which was then stirred for 1 h at 50 °C. After cooling the reaction mixture to ambient temperature, a mixture of SnCl2·2H2O (8.2 g, 3.6 mmol) in 10% HCl (15 mL) was added and the mixture was stirred for additional 1.5 h, after which it was poured into ice water. The mixture was extracted with diethyl ether twice and the combined organic phases were concentrated to obtain the crude product. Further purification was carried out by column chromatography on silica gel using petroleum ether as the eluent to obtain pure compound (4) as pale yellow viscous liquid solidifies on cooling (1.5 g, yield 50%). 1H NMR (CDCl3, 300 MHz), δ (ppm): 7.67(d, 2H), 7.47 (d, 2H), 7.42 (d, 2H), 7.02 (d, 2H), 2.94 (d, 4H), 1.68 (m, 2H), 1.49–1.35 (br, 16H), 1.0–0.92 ppm (m, 12H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 145.19, 139.06, 137.51, 136.44, 127.61, 127.30, 125.30, 124.03, 123.29, 41.55, 34.33, 32.53, 29.42, 25.83, 23.06, 15.67, 11.00.
2.2.5. Poly[4,8-bis(5-(2-ethylhexyl)selenophene-2-yl)benzo[1,2-b:4,5-b′] dithiophene-diyl -alt-[4,7-(5-(heptadecan-9-yl)-5H-thieno[3,4-c]pyrrole4,6-dione) (P2) Monomer 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)selenophen-2yl)benzo[1,2-b;4,5-b′] dithiophene (5) (300 mg, 0.3 mmol) and 1,3-dibromo-5-(heptadecan-9-yl)-5H-thieno[3,4-c]pyrrole-4, 6-dione (165 mg, 0.3 mmol) (7) were mixed in 4 mL of toluene and 1 mL of DMF to the pressure tube. After being purged with argon for 5 min, Pd(PPh3)4 (15 mg) was added as the catalyst, and the mixture was then purged with argon for 25 min. The reaction mixture was stirred and heated at 110 °C for 24 h. Then the reaction mixture was cooled to room temperature, and the polymer was precipitated by addition of 50 mL methanol, filtered and dried. Polymer was redissolved in chlorobenzene and precipitated in 50 mL methanol. The precipitate was then filtered and subjected to Soxhlet extraction with methanol, acetone, hexanes, and chloroform. The polymer was recovered as solid from the chloroform fraction by precipitation from methanol. The solid was dried under vacuum for 24 h at 40 °C. The yield and elemental analytical results of the polymer are as follows. Yield (240 mg, 76%). Mn = 23.5 kg/Mol, Mw = 49.1 kg/mol, PDI (Mw/Mn) = 2.08.1H NMR (CDCl3, 300 MHz), δ (ppm): 7.50 (br, 2H), 7.42 (br, 2H), 6.80 (br, 2H), 3.4 (br, 1H), 2.86 (br, 4H), 1.80–1.75 (br, 2H), 1.7 (2H),1.60–1.58 (br, 2H), 1.28–1.1 (br, 40 H), 0.90 (br, 12H), 0.8 (br, 6H). Elemental analysis calculated, C, 64.81; H, 6.77; N, 1.33; S, 9.11; found C, 64.76; H, 6.68; N, 1.27; S, 9.02.
2.2.3. 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)selenophene-2-yl) benzo[1,2-b:4,5-b′] dithiophene (5) A solution of compound (4) (1.5 g, 2.2 mmol) in THF (40 mL) at 0 °C was placed in a 100 mL argon purged flask, and then n-butyl lithium (1.6 M, 4.1 mL, 6.6 mmol) was added. The reaction mixture was then stirred for 2 h at ambient temperature. Subsequently, chlorotrimethylstannane (1.0 M in hexane, 8.9 mL, 8.8 mmol) was added and the mixture was stirred for an additional 1 h at ambient temperature. Then, the mixture was poured into ice water. Extracted by diethyl ether and the combined organic phase were concentrated to obtain crude compound. Further purification was carried out by recrystallization using ethanol to obtain the pure compound (5) as a light-yellow solid (1.4 g, yield 63%).1H NMR (CDCl3, 300 MHz), δ (ppm): 7.52 (s, 2H), 7.24 (d, 2H), 6.88 (d, 2H), 2.75 (d, 4H), 1.54 (m, 2H), 1.28–1.1 (br, 16 H), 0.80 (m, 12H), 0.21 (s, 18H). 13C NMR (CDCl3, 100 MHz), δ (ppm): 145.39, 143.28, 142.24, 138.00, 137.31, 131.18, 127.54, 125.30, 122.41, 41.57, 34.25, 32.52, 28.97, 25.81, 23.04, 15.42, 10.99, −8.34.
3. Results and discussion Conjugated polymers P1 and P2 are synthesized by Pd(PPh3)4 catalyzed Stille copolymerization reaction using the monomers, 2,6-Bis (trimethyltin)-4,8-bis(5-(2-ethylhexyl)selenophene-2-yl)benzo [1,2b:4,5-b′]dithiophene (SeBDT) as a donor, 4,7-bis(5-bromo-4-(2-ethylhexyl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine (DTPyT) or dibromo-5-(heptadecan-9-yl)-5H-thieno[3,4-c]pyrrole-4,6-dione (TPD) as acceptors, respectively. Synthetic route for donor monomer SeBDT is shown in Scheme 1, and the copolymerization is illustrated in Scheme 2. Ethylhexyl alkyl side chain group is introduced on selenophene substituted benzodithiophene as well as on dithienothiadiazole[3,4-c] pyridine and octylnonyl chain is incorporated on thienopyrroledione monomers to ensure the solubility of the corresponding polymers. After the completion of polymerization reaction, the crude polymers were collected by precipitation in methanol and filtration. Further purification is carried by Soxhlet apparatus, using methanol, acetone, hexane and chloroform successively to remove the by-products. The final polymers were obtained by evaporation of chloroform and precipitating in methanol. Polymers are filtered and dried in vacuum. Thus, the obtained polymers P1 and P2 are readily soluble in common organic solvents such as chloroform, chlorobenzene and dichlorobenzene, indicating easy processability during fabrication of organic electronic devices. The number average molecular weights (Mn) and dispersity (PDIs) of the copolymers were determined by the gel permeation chromatography (GPC) analysis with a polystyrene standard calibration in chloroform eluent, and were in the range of 23.5–26.2 kg mol−1 with a dispersity of 2.08–2.40 (Table 1). The structures of all the products were confirmed by 1H NMR spectroscopy. In addition, the polymers were characterized by their elemental analysis.
2.2.4. Poly[4,8-bis(5-(2-ethylhexyl)selenophene-2-yl)benzo[1,2-b:4,5-b′] dithiophene–diyl-alt-[4,7-bis(4-(2-ethylhexyl)thiophen-2-yl)-[1,2,5] thiadiazolo[3,4-c]pyridine](P1) Monomer 2,6-Bis(trimethyltin)-4,8-bis(5-(2-ethylhexyl)selenophen2-yl)benzo[1,2-b; 4,5-b′]dithiophene (5) (300 mg, 0.3 mmol) and 4,7bis(5-bromo-4-(2-ethylhexyl)thiophen-2-yl)-[1,2,5] thiadiazolo [3,4-c] pyridine (6) (204 mg, 0.3 mmol) were mixed in 4 mL of toluene and 1 mL of DMF to the pressure tube. After being purged with argon for 5 min, Pd(PPh3)4 (15 mg) was added as the catalyst, and the mixture was then purged with argon for 25 min. The reaction mixture was stirred and heated at 110 °C for 24 h. Then the reaction mixture was cooled to room temperature, and the polymer was precipitated by addition of 50 mL methanol, filtered and dried. Polymer was redissolved in chlorobenzene and precipitated in 50 mL methanol. The precipitate was then filtered and subjected to Soxhlet extraction with methanol, 642
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Thermogravimetric analysis (TGA) of polymers P1 and P2 was investigated to determine the thermal stability. Fig. 1 shows the TGA chart, where the onset temperature with 5% weight loss of P1 is above 370 °C and for P2, it is around 280 °C (Table 1). This impressive thermal stability is comparable to alkyl substituted BDT polymers, which are recommended for organic solar cell and organic field effect transistor applications. Interestingly, polymer P1 containing DTPyT exhibited high thermal stability due to the presence of dithienothiadiazole[3,4-c] pyridine moiety as acceptor part in the polymer backbone as compared to the polymer P2 containing TPD moiety as an acceptor. A similar thermal stability trend is observed for alkyl substituted BDT based polymers [12,14]. The thermal stability of these polymers is adequate for their applications in optoelectronic devices. The absorption spectra of the polymers in chloroform solution and in the thin films form are shown in Fig. 2. Optical data including the maximum absorption peak wavelengths (λmax) and the optical band gap (Egopt) are summarized in Table 2. The entire absorption spectra feature broad absorption bands from 350 nm to 900 nm long wavelength regions. The absorbance bands in chloroform solution for P1 were observed at 440 nm and 660 nm, while those of P2 were found at 557 nm and 614 nm. The higher energy absorbance are attributed to localized π-π* transitions while the lower energy bands were associated with an intramolecular charge transfer (ICT) between the donor and acceptor similar to those characterized by Jespersen et al. [23,24]. The polymer films were slightly red-shifted by 14–40 nm, which is most likely due to the high coplanarity or enhanced intermolecular electronic interactions in the solid state, leading to stability of a lower energy excited state. The optical band gaps (Egopt) for P1 and P2 determined from their film absorption edges (λonset) are about 1.53 and 1.84 eV, respectively. As shown in Fig. 3, the onset oxidation potentials (Eox) of P1 and P2 are 0.47 and 0.49 V, respectively versus Ag/AgCl reference electrode. Using ferrocene reference value of −4.8 eV below the vacuum level [24], the HOMO energy levels of the polymers were calculated as EHOMO = −(Eox − EFc + 4.8) (eV), wherein EFc = 0.25 eV is the potential of the internal standard of the ferrocene/ferrocenium ion (Fc/Fc+) couple. Thus, the HOMO levels were determined to be −5.02 eV for P1 and −5.04 eV for P2. Based on the optical band gap (Egopt) values and HOMO energy levels, the LUMO energy levels of P1 and P2 were estimated to be −3.49 and −3.20 eV, respectively, The results are summarized in Table 2. Compared to P1 (HOMO = −5.02 eV), P2 shows a deeper HOMO energy level of −5.04 eV, which indicates the presence of strong π-π stacking among the polymer backbone and formation of ordered arrangements in their solid films. In comparison, both polymers show almost similar optical properties. In contrast, in the case of polymer films, P2 shows a clear red shift than does P1. Note that the thermal, optical and electrochemical properties of selenophene substituted benzo[1,2-b:4,5-b′]dithiophene (SeBDT) polymers are comparable to that of alkyl substituted polymers (Supporting Information Table S1 and Table S2) employed in organic electronics. This suggests the polymers P1 and P2 as potential candidates for organic electronic applications.
electronics. Additional modifications to the polymer structure to explore these materials in organic electronic application are currently underway. Acknowledgement One of the author Shubhangi Khadtare is thankful to Scientific and Engineering Research Board for overseas postdoctoral Award, SERB No. SB/OS/PDF-107/2015-16. Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx. doi.org/10.1016/j.dyepig.2017.11.026. References [1] Heeger AJ. Semiconducting and metallic polymers: the fourth generation of polymeric materials. Angew Chem, Int Ed 2001;40:2591–611. [2] Lewis NS. Toward cost-effective solar energy use. Science 2007;315:798–801. [3] Bijleveld JC, Zoombelt AP, Mathijssen SG, Wienk MM, Turbiez M, de Leeuw DM, et al. Poly(diketopyrrolopyrrole-terthiophene) for ambipolar logic and photovoltaics. J Am Chem Soc 2009;131:16616–7. [4] Yu G, Gao J, Hummelen JC, Wudl F, Heeger AJ. 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4. Conclusion In summary, we report the synthesis and characterization of two novel selenophene substituted benzo[1,2-b:4,5-b′]dithiophene (SeBDT) containing conjugated polymers (P1 and P2) consisting of dithienothiadiazole [3,4-c]pyridine (DTPyT) and thieno[3,4]pyrroledione (TPD) acceptors synthesized through Stille polymerization reaction. Polymers exhibited good thermal stability and broad absorption bands. The HOMO and LUMO energy levels of the polymer P1 and P2 were found to be −5.02, −5.04 eV and −3.49, −3.20 eV respectively. Synthesized conjugated polymers are highly soluble in common organic solvents, which is a critical factor for fabricating electronic device. The photophysical properties and optoelectronic studies of the synthesized polymers suggest these materials to be the promising candidates for the application in organic 643