J Fluoresc DOI 10.1007/s10895-017-2040-3
ORIGINAL ARTICLE
Optical, Electrochemical and Thermal Studies of Conjugated Polymers Synthesized by Eutectic Melt Reaction Chinna Bathula 1,2 & Kezia Buruga 3 & Youngjong Kang 1,3 & Imtiyaz Ahmed M. Khazi 4
Received: 7 October 2016 / Accepted: 3 February 2017 # Springer Science+Business Media New York 2017
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Keywords Benzodithiophene . Thienithiophene . Thienopyrroledione . Eutectic reaction . Optoelectronic studies
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Introduction Solvent-free synthetic protocol for organic synthesis is gaining importance as a tool for the synthesis of a wide variety of useful and important compounds, with the number of reactions conducted under these conditions increasing. Initially, conventional methods have been utilized for solvent-free synthesis, but lately there has been a shift in utilizing nonconventional energy sources, such as microwaves, ultrasound and mechanochemical mixing to increase the efficiency of the reactions. Solid state melt reaction is an economically attractive and environmentally acceptable synthetic tool for the construction of a wide range of structurally diverse heterocyclic motifs without using toxic, flammable, expensive, and hazardous organic solvents [1–4]. Recently, the catalyst-free multi component reactions gained interest as alternative to the classical organic synthesis [5]. Taking into account that the most common procedures require large amounts of organic solvents, the presence of strong acidic reagents, long reaction times and complex work-up, the aim of this work was to study more ecological and economic conditions, as green chemistry procedures, for the synthesis of these widespread ingredients with special interest focused on solvent-free methodologies [6–9]. It is well known that, in organic synthesis, avoiding the solvent leads to enhanced yields, milder conditions, increased safety and cost reduction [10]. In Literature there are also reports on the synthesis of potential cosmetic ingredients in dry media [11, 12]. Conjugated low band gap polymers containing alternating electron rich donor (D) and electron-poor acceptor (A) units have emerged as a popular approach to
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Abstract This paper reports on the synthesis of a novel donor–acceptor conjugated polymers, P1 and P2 by solvent free eutectic melt polymerization reaction. Triisopropylsilylethynyl(TIPS) substituted benzo[1,2-b:4,5-b ′]dithiophene(BDT) is used as donor, thienithiophene(TT) and thienopyrroledione(TPD) are utilized as acceptors for demonstrating eutectic polymerization. The most important fact in the solvent-free reaction between solid reactants actually proceeds through bulk liquid phases. Such liquid phases are possible due to the formation of eutectics between the reactants and product(s) and any evolution of heat. Naphthalene is explored in this reaction for forming eutectics with the reactants, resulting in desired polymers. Thermal stability, optical and electrochemical properties of these polymers were determined. Optical band gaps of the polymers were found to be 1.58 and 1.65 eV. Electrochemical studies by cyclic voltametry experiment revealed HOMO and LUMO energy levels to be −5.22, −5.60 eV, and −3.76, −4.16 eV, respectively. The polymers were thermally stable up to 285– 400 °C. Thermal, optical and electrochemical studies indicated these materials to be promising candidates in organic electronic applications.
* Chinna Bathula
[email protected]
1
Department of Chemistry, Research Institute for Natural Sciences Hanyang University, Seoul 133-791, Republic of Korea
2
Institute of Material Design and Institute of Nanoscience and Technology, Hanyang University, Seoul 133-791, Republic of Korea
3
Department of Chemical Engineering, National Institute Technology, Surathkal, Karnataka 575025, India
4
Dapartment of Chemistry, Karnatak University, Dharwad 580 003, India
J Fluoresc Scheme 1 Synthetic route for the copolymers
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to cast engineering alloys, [20] but they were barely employed in polymerization. To the best of our knowledge, there are no reports on the synthesis of organic semiconducting polymers by naphthalene assisted eutectic polymerization. To demonstrate the eutectic polymerization concept, monomers used in our experiments includes triisopropylsilylethynyl(TIPS) substituted benzo[1,2-b:4,5-b ′]dithiophene(BDT), thienithiophene(TT) and thienopyrroledione(TPD). In the present study, the crystalline solid with moderately high vapor pressure and low melting temperature such as naphthalene, is utilized to form the eutectic mixture. The importance of solvent-free synthesis using nonconventional method is highlighted in the present article. Thermal stability, optical and electrochemical properties of these polymers were determined. Optoelectronic studies of the synthesized polymers indicated these materials to be promising candidates in organic electronics.
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improve the device efficiency and operational lifetime along with fluorescence properties [13, 14]. The building blocks benzo[1,2-b:4,5-b]dithiophene (BDT) [15, 16], naphthodithiophene (NDT), [17, 18] and 4,7-dithien-2-yl2,1,3-benzothiadiazole (DTBT) [19] have been proven to be excellent D-A units among the high performing materials. Herein, we report the synthesis of organic semiconducting polymers by eutectic polymerization reaction. We have utilized the strong melting temperature depression induced by eutectic mixtures for polymerization at moderately low temperature. Due to the lowered melting temperature, the polymerization by eutectic melting is highly applicable for a variety of organic molecules which originally have high melting temperature or/and insoluble in most organic solvents. Eutectic systems, which are mixtures of chemical compounds or elements that melt and solidify at lower temperature than any of pure ingredients, have long been utilized in metallurgy Fig. 1 Structures of copolymers
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Experimental Materials Tetrakis(triphenylphosphine)palladium and naphthalene were purchased from Aldrich. All chemicals were used without further purification. The monomers 2,6-Bis(trimethyltin)4,8-bis(triisopropylsilylethynyl)-benzo[1,2-b:4,5-b ′]dithiophene [21], dodecyl 4,6-dibromothieno[3,4-b]thiophene-2-carboxylate [22], and 1,3-dibromo-5-(heptadecan-9yl)-5H–thieno[3,4-c]pyrrole-4,6-dione [23] were prepared by previously described methods.
hexanes to remove oligomers and catalyst residues followed by extraction with chloroform. The polymers were obtained by evaporation of chloroform and precipitating in methanol. Finally, the polymer is filtered and dried in vacuo at 40 °C. Dark blue shining material is obtained in the yield of 78%. 1H NMR (CDCl3, 300 MHz, δ/ppm): 7.9 (br, 1H), 7.72 (br, 1H), 7.68 (br, 1H), 4.4 (br, 2H), 1.80 (br, 2H), 1.2–1.1 (br, 60H), 0.82 (br 3H). Anal.calcd: C, 68.10; H, 7.84; S, 14.26; found C, 68.08; H, 7.80; S, 14.28.
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Synthesis of poly[4,8-bis(triisopropylsilylethynyl) benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl-alt-[5-(heptadec an-9-yl)-5H–thieno[3,4-c]pyrrole-4,6-dione] (P2)
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Synthesis of Polymers
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The synthesized compounds were characterized with 1H NMR and 13C NMR spectra obtained using a Bruker DPX300 NMR spectrometer. UV-visible analysis was performed using a Lambda 20 (Perkin Elmer) diode array spectrophotometer. The number and 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.
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Material Characterization
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All reactions were carried out in an argon atmosphere using the usual Schlenk techniques. Synthesis of poly[4,8-bis(triisopropylsilylethynyl) benzo[1,2-b:4,5-b ′]dithiophene-2,6-diyl-alt-[4,6-(2-dodecyl-thieno[3,4-b] thiophene-2-carboxylate] (P1) 2,6-Bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)benzo[1,2-b:4,5-b′]dithiophene (262 mg, 0.3 mmol), dodecyl 4,6-dibromothieno[3,4-b]thiophene-2-carboxylate (153 mg, 0.3 mmol) and Pd(PPh3)4 (14 mg, 0.05 eq.) and naphthalene (400 mg, 3.0 mmol) were added to a 20 mL glass viol with septum under argon atmosphere. The polymerization was carried out at 110 °C under argon protection. Naphthalene forms eutectic mixture with the starting material and acts as a melt liquid. After 12 h, the reaction mixture was cooled to room temperature. Methanol (15 mL) is added to the reaction mixture and vigorously stirred over night. The polymer fibres were collected by by filtration and reprecipitation from methanol. The polymer was dissolved in chlorobenzene and precipitated again into methanol. The polymer precipitate was then subjected to Soxhlet extraction with methanol, acetone,
The polymer P2 was synthesized with the procedure described as in for P1. The co-polymerization of the monomers 2,6bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)-benzo[1,2b:4,5-b′]dithiophene (262 mg, 0.3 mmol), 1,3-dibromo5-(heptadecan-9-yl)-5H–thieno[3,4-c]pyrrole-4,6-dione (177 mg, 0.3 mmol) and Pd(PPh3)4 (14 mg, 0.05 eq.) and naphthalene (400 mg, 3.0 mmol) gave P2 (67%) of dark red shining material. 1H NMR (CDCl3, 300 MHz, δ/ppm7.75 (br, 1H), 7.60 (br, 1H), 4.2 (br, 2H), 1.78 (br, 2H), 1.2–1.1 (br, 74H), 0.82 (br 6H). Anal.calcd: C, 71.04; H, 8.74; N, 1.43; S, 9.81; Found C, 70.96; H, 8.72; N, 1.40; S, 9.74;
J Fluoresc Table 1
Physical properties of the polymers
Polymer Mna [kg/mol] Mw a [kg/mol] PDI
Yield [%] Td b [oC]
P1
26.2
60.3
2.30 78
285
P2
31.5
91.3
2.89 67
400
a
The molecular weights were determined by using gel permeation chromatography (GPC) against polystyrene standards in chloroform eluent
b
Temperature resulting in 5% weight loss based on initial weight
Results and Discussion Synthesis and Characterization of the Polymers Fig. 3 UV-Vis absorption spectra of the polymer P1 in chloroform solution and thin film state
and were in the range of 26.2–31.5 kg mol−1 with a poly dispersity index of 2.30–2.89 (Table 1).
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Thermal Properties
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The thermal stability of polymers P1 and P2 was investigated by using thermogravimetric (TGA) analysis with a heating rate of 10 °C min−1 under nitrogen. Figure 2 shows the TGA where the onset temperature with 5% weight loss (Td) of P1 is above 285 °C and for P2 is around 400 °C. The thermal stability of these polymers was adequate for their applications in optoelectronic devices (Table 1).
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Conjugated polymers P1 and P2 are synthesized by Pd(PPh3)4 catalyzed eutectic polymerization using the monomers, 2,6bis(trimethyltin)-4,8-bis(triisopropylsilylethynyl)-benzo[1,2b:4,5-b′]dithiophene, dodecyl-4,6-dibromothieno[3,4-b]thiophene-2-carboxylate, and 1,3-dibromo-5-(heptadecan-9-yl)5H–thieno[3,4-c]pyrrole-4,6-dione respectively. Naphthalene is utilized to form eutectic mixture with the monomers. Structures of all the polymers have been depicted in Fig. 1. Synthetic route for the copolymerization is illustrated in Scheme 1. Triisopropylsilylethyny(TIPS) group is introduced on benzo[1,2-b:4,5-b]dithiophene(BDT), dodecyl alkyl chain is substituted on thienithiophene(TT) and heptadecanyl chain on thienopyrroledione(TPD) to promote solubility of the corresponding polymer. The crude polymers were collected by precipitation in methanol, and extracted with methanol and acetone using Soxhlet apparatus to remove the byproducts. The number average molecular weights (Mn) and polydispersity indices (PDIs) of the copolymers are determined by the gel permeation chromatography (GPC) analysis with a polystyrene standard calibration in chloroform eluent
Fig. 2 TGA plots for the polymers, obtained with a heating rate of 10 °C min−1 under an inert atmosphere
Optical Properties To determine the optical properties of P1 and P2, thin films were prepared on quartz plates. Prior to deposition of polymer, plates were thoroughly cleaned with deionized water,
Fig. 4 UV-Vis absorption spectra of the polymer P2 in chloroform solution and thin film state
J Fluoresc Table 2 Optical and electrochemical properties of the polymers
Polymer
λmaxabs,Sol a [nm]
λmaxabs,Film a [nm]
HOMO b [eV]
LUMO c [eV]
Egopt d [eV]
P1 P2
620 615
650 680
−5.30 −5.25
−3.72 −3.60
1.58 1.65
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
value of −4.8 eV below the vacuum level, [25] the HOMO energy levels of the polymers were calculated as E HOMO ¼ −ðE OX −E Fc þ 4:8ÞðeVÞ
ð1Þ
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wherein EFc is the potential of the internal standard, the ferrocene/ferrocenium ion (Fc/Fc+) couple. The value of EFc,
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chloroform and acetone followed by drying in the oven. The semiconducting polymer layer in chloroform solution was spin-coated with concentration of 6 mg mL−1. The films are dried at 40 °C. The absorption spectra of the polymers in chloroform solution and in the thin films for polymers P1 and P2 are shown in Figs. 3 and 4. 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 300 nm to 800 nm long wavelength regions. The absorbance bands in chloroform solution for P1 were observed at 450 nm and 620 nm while those of P2 were seen at 580 and 615 nm. The polymer films were slightly red-shifted by 20–50 nm most likely due to high coplanarity or enhanced intermolecular electronic interactions in the solid state such that lead to stability of a lower energy excited state. Finally, the optical band gaps (Egopt) for P1 and P2 determined from their film absorption edges (λonset) were about 1.58 and 1.65 eV, respectively. The higher energy absorbances were 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. [24, 25].
Electrochemical Properties Cyclic voltammetry (CV), was used to investigate the electrochemical behaviour of the conjugated polymers and to estimate their HOMO and LUMO energy levels. For the electrochemical properties of P1 and P2 as 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 used as electrolyte in acetonitrile solution with scan rate 50 mV s−1. Based on CV, the onset oxidation and reduction potentials of the voltammogram correspond to HOMO and LUMO energy levels, respectively. As shown in Fig. 5, the onset oxidation potentials (Eox) of P1 and P2 are 0.75 and 0.70 eV, respectively versus Ag/AgCl reference electrode. Using ferrocene reference
Fig. 5 Cyclic Voltammograms of the thin film of the copolymers P1 and P2
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References 1.
2.
E LUMO ¼ E HOMO þ E g opt ðeVÞ
ð2Þ
The LUMO values are determined to be −3.72 eV and −3.60 eV respectively such that the energy gap between the HOMO and LUMO to be 1.58 eV and 1.65 eV for P1 and P2 respectively. The electrochemical properties are summerized in Table 2. These data determined by uv-visible and cyclic voltametry experiments shows that, presence of the longer alkyl chain in P2 resulted in a lower energy HOMO as well as LUMO while maintaining a lower overall band gap. Compared to P2 (HOMO = −5.25 eV), P1 shows a deeper HOMO energy level of −5.30 eV indicates the presence of strong π-π stacking among the polymer backbone and formation of ordered arrangements in their solid films. Both polymers showed similar optical properties and gives clear red shift in films.
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which is determined under the same experimental conditions is approximately 0.25 eV vs Ag/Ag+. The HOMO energy levels of the polymer are found to be −5.30 eV for P1 and −5.25 eV for P2. From this value, the LUMO energy levels of the polymer is obtained using eq. (2)
Conclusions
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In conclusion, 2 examples of novel donor–acceptor types of conjugated polymers have been syntheized based on triisopropylsilylethynyl(TIPS) substituted benzo[1,2-b:4,5b]dithiophene(BDT) donor, thienithiophene(TT) and thienopyrroledione(TPD) acceptors for demonstrating naphthalene assisted eutectic polymerization. Both the polymers showed improved solubility and high molecular weights. Optical band gaps of the polymers were found to be 1.58 and 1.65 eV. Electrochemical studies of conjugated polymers (P1and P2) revealed that the HOMO and LUMO energy levels to be −5.30, −5.25 eV, and −3.72, −3.60 eV, respectively. The optoelectronic studies suggests the synthesized copolymers to be the promising candidates for the application in organic electronics. For the future research on solid state melt polymerization for synthesis of conjugated polymers, the systematic variation of similar building blocks as applied in this study holds great promise for comprehensive correlations of electronic properties with the molecular structure. Additional modifications to the naphthalene assisted eutectic polymerization are currently under study in our laboratory.
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Acknowledgements We gratefully acknowledge the support by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1401-05.
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