Abstract
Two new copolymers, P1 and P2, composed of 4[4-(alkoxy)phenyl]-2, 6-bis(4-bromophenyl)pyridine and bithiophene units, have been synthesized via Suzuki cross-coupling reactions. 4[4-(alkoxy)phenyl]-2, 6-bis(4-bromophenyl)pyridines were synthesized starting from condensation reaction of 4-bromoacetophenone and 4-hydroxy benzaldehyde, and subsequent alkoxylation of hydroxyl groups. All of the polymers and intermediates were characterized using FTIR and NMR spectroscopies. The synthesized polymers exhibit good solubility in common organic solvents. The maximum absorption peak for P1 and P2 was 430 and 420 nm, respectively. The optical band gap energy of the polymers was determined by absorption onset and found to be 2.17 eV for P1 and 2.13 eV for P2.
1. Introduction
The research for conjugated polymers in organic electronic and optoelectronic applications with highly stable, highly conductive, and easily processable, has led to extensive studies over the past two decades.[Citation1,Citation2]
In this area, many efforts have been taken to construct and design new polymers with enhanced properties, such as incorporating flexible subtsituents, using electron donating and/or electron-withdrawing substituents, and hybridizing with different conjugated molecules, such as various types of phenylenes.[Citation3–Citation11]
One of the important parts of these researches is tuning HOMO and LUMO levels of polymers by changing the electron donating or withdrawing nature of side chain groups in polymer backbone. Electron donating groups, raising the HOMO levels of polymers and electron acceptors pulling down the LUMO, and therefore, both reduced the final band gap.[Citation12–Citation19] Another important factor of using a polymer in solar cell applications is its solubility in common organic solvents which make it possible to process easily.Citation[20]
The longer side chains caused better solubility of polymers; however, this will influence other phenomena like its conduction. The longer chain have lower conductance, because the electron transfer is less and caught effect will be more.[Citation21–Citation24] In polymers, if the backbone chains are close together, electrons and holes can be transferred from one another. So, long chains have both advantage and disadvantage and the length of these chains must be optimized.
Usually, in the design of new highly electron/hole conductive polymers, meta-linkages between conducting segments are generally excluded because they interrupt conjugation. These meta-linkages have usually been introduced in order to reduce conjugation in a tunable manner, for example, in synthesizing polymers with blue emission.[Citation25–Citation27]
In fact, it appears to be widely accepted that meta-linkages in polymers are something to be avoided if one wishes to produce systems with high conductivity that is generally associated with delocalized carriers. However, there are some reports about using meta-linkages, to produce highly conductive polymers.[Citation28–Citation31]
Polythiophene and its derivatives are the most widely studied conducting polymers.[Citation32,Citation33] Polymers, containing special functional groups like: chromophores, emitters, electrochromophores, ligands, etc. electronically coupled with the conjugated main chain are particularly interesting and important, because of the mutual interactions of the functional group and the conducting chain.[Citation34,Citation35]
Due to high electron affinity, pyridine-containing conjugated polymers have been shown to be promising candidates for light-emitting device (LED).[Citation36,Citation37] The homopolymer of pyridine (Ppy) act as blue emitting material and the other pyridine-containing copolymers have been shown to be highly luminescent.[Citation38–Citation41] The reactivity of pyridine moiety for N-protonation, N-oxidation, and quaternization with RX can also modify the optical and electrical properties.Citation[42] Also in comparison to phenylene-based polymers, they are more resistant to oxidation and show better electron-transporting properties due to higher electron affinity. The capability of using low work function metals, like Al or ITO, makes the pyridine-containing polymers the excellent candidates for polymer LEDs.
Synthesis of polymers containing both thiophene and pyridine rings could be done via some strategies. One type is making a desired thiophene-pyridine-containing monomer via coupling reactions Citation[43] and then its polymerization. The other route is making thiophene-pyridine-containing monomer via modified chichibabin pyridine synthesis [Citation44–Citation46] and then its polymerization via coupling.
In continuation of our works on synthesis of pyridine-containing monomers via modified chichibabin synthesis and related polymers,[Citation47–Citation49] now we used a third route for synthesizing two new copolymers composed of thiophene and pyridine groups via coupling between a synthesized pyridine-containing dibromo compounds and bithiophene with diboronic ester substituents via Suzuki coupling.
All of the compounds and intermediates were characterized using FTIR, 1H NMR, and CHN elemental analysis. The properties of synthesized polymers like thermal stability, molecular weight, HOMO, and LUMO levels were investigated.
2 Experimental
2.1 Materials
p-Bromoacetophenone, p-hydroxybenzaldehyde, and tetrakis (triphenyl phosphine)Pd(0) were obtained from Merck Chemicals. 2, 2′-Bithiophene-5, 5′-dibronic acid bis (pinacol) ester was purchased from Aldrich Chemicals. The other additives and solvents were obtained from Merck Chemicals. Solvents should be degassed using nitrogen atmosphere before use.
2.2 Instrumentation
1H NMR spectra were recorded on a Bruker Avance 400 spectrometer. Molecular weight was obtained using a Knauer gel permeation chromatography (GPC) equipped with Smartine Pump 1000, a differential refractive index detector (Smartin Ri 2300), and PL Gel 10 μm column. THF was used as solvent with flow rate of 1 ml/min at room temperature. GPC was calibrated with standard polystyrene with polydispersity index of 1.09. FTIR spectra were recorded on a JASCO FTIR-6300 spectrometer using KBr pellets. Thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed on a Mettler TG5 system under nitrogen atmosphere at a heating rate of 10 °C/min. Ultraviolet–visible (UV/Vis) absorption spectra were recorded on a JASCO V-670 spectrophotometer. The electrochemical measurements of the copolymer was carried out on a potentiostat/galvanostat Autolab PGSTAT30 with a platinum electrode at a scan rate of 50 mV/s against Ag/AgCl reference electrode with nitrogen-saturated solution of 0.1 M tetra-butyl-ammonium hexafluorophosphate in acetonitrile. The polymers and TBAPE were dissolved in DMF. The elemental analysis was carried out by a Leco, CHNS-932.
2.3 Monomer synthesis
2.3.1 Synthesis of 4(4-hydroxyphenyl)-2, 6-bis (4-bromophenyl) pyridine (R1)
In a round-bottomed 100 mL flask, a solution of p-bromo acetophenone (11.94 g, 60 mmol), p-hydroxy benzaldehyde (3.66 g, 30 mmol), and ammonium acetate (40 g) in acetic acid (80 mL) was prepared. The mixture was heated to reflux for 4 h and cooled to room temperature, and a yellow product was isolated by filtration and washed with methanol followed by recrystallization from DMF. The solid was dried in vacuum at 130 °C for 24 h. A light yellow powder (R1) was obtained.
Yield: 70%; mp: 283 °C
FTIR (KBr, cm−1): 3151, 1601, 1514, 1487, 1207, 1072, 821
CHNS (C23H15Br2NO): C 56.92% (57.41%, calculated), H, 3.62% (3.14%), N, 3.00% (2.91%).
1H NMR (DMSO-d6, ppm): 9.89 (OH, 1H), 8.29–8.26 (aromatic, 4H), 8.17 (aromatic, 2H), 7.94–7.91 (aromatic, 2H), 7.74–7.72 (aromatic, 4H), 6.95–6.93 (aromatic, 2H).
2.3.2 Synthesis of 4[4-(isopentyloxy) phenyl] 2, 6-bis (4-bromophenyl) pyridine (R2)
In a 50 mL one-neck round-bottom flask, a mixture of R1 (2.2 g, 4.56 mmol) and NaOH (0.14 g, 3.51 mmol) in DMF (20 mL) was prepared. The mixture was heated to reflux for 4 h and then 1-boromo-3-methyl butane (0.441 mL, 3.51 mmol) was added and continued for 2 h. Finally, the mixture was cooled and methanol was added to precipitate the product. The mixture was filtered and washed with methanol. A brown product was obtained after recrystallization from chloroform. The solid was dried in vacuum at 40 °C for 24 h.
Yield: 60%; mp: 123 °C
FTIR (KBr, cm−1): 2949, 1598, 1513, 1254, 822
CHN (C28H25Br2NO): C, 61.00% (61.00%, calculated), H, 4.84% (4.57%), N, 2.65% (2.54%)
1H NMR (CDCl3, ppm): 8.07–8.05 (aromatic, 4H), 7.84–7.71 (aromatic, 2H), 7.69–7.65 (aromatic, 6H), 7.07–7.05 (aromatic, 2H), 4.11–4.08 (–OCH2–, 2H), 1.92–1.87 (–CH–, 1H), 1.76–1.73 (–CH2–, 2H), 1.03–1.01 (CH3, 6H).
2.3.3 Synthesis of 4[4-(decyloxy) phenyl] 2, 6-bis (4-bromophenyl) pyridine (R3)
This was synthesized according to the same procedure used for synthesis of R2. Only 1-boromodecane was used instead of 1-boromo-3-methyl butane.
Yield: 56%; mp: 77 °C
FTIR (KBr, cm−1): 2919, 1598, 1513, 1256, 1007, 825
CHN (C33H35Br2NO): C, 63.14% (63.78%, calculated), H, 5.85% (5.68%), N, 2.28% (2.25%)
1H NMR (CDCl3, ppm): 8.07–8.05 (aromatic, 4H), 7.84–7.71 (aromatic, 2H), 7.69–7.65 (aromatic, 6H), 7.07–7.05 (aromatic, 2H), 4.07–4.04 (–OCH2–, 2H), 1.87–1.31 (–CH2–, 16H), 0.93–0.89 (–CH3, 3H).
2.4 Polymer synthesis
2.4.1 Polymer from reaction of 4[4-(isopentyloxy)phenyl] 2,6-bis(4-bromophenyl)pyridine with 2,2′-bithiophene-5,5′-dibronic acid bis(pinacol)ester (P1)
In a 100 mL two-neck round-bottom flask, a mixture of R2 (0.137 g, 0.25 mmol), 2,2′-bithiophene-5,5′-dibronic acid bis(pinacol)ester (0.105 g, 0.25 mmol), and tetrakis(triphenyl phosphine)pd(0) (0.003 g) in THF (20 mL) was prepared. The mixture was heated to reflux, and after 20 min, an aqueous solution of K2CO3 (4 M, 3 mL) was added under nitrogen. The mixture was refluxed for 36 h and then bromo benzene was added for end capping. The reaction was allowed to reflux for 24 h and then methanol was added to mixture. The precipitate was filtered and washed with deionized water and methanol. The dark green polymer was obtained after drying in vacuum at 70 °C.
FTIR (KBr, cm−1): 2920, 1628, 1397, 1258, 1008, 833, 697
1H NMR (CDCl3, ppm): 8.25–7.11 (aromatic, 18H), 4.03 (–OCH2–, 2H), 1.51–0.93 (aliphatic, 9H)
GPC: Mw = 28,010 g/mol, Mn = 23,130 g/mol, PDI = 1.21
Tg (DSC) = 190 °C
2.4.2 Polymer from reaction of 4[4-(decyloxy) phenyl] 2, 6-bis (4-bromophenyl) pyridine with 2, 2′-bithiophene-5, 5′-dibronic acid bis (pinacol) ester (P2)
This polymer was synthesized according to method reported for synthesis of P1. Only R3 was used instead of R2 in this section.
FTIR (KBr, cm−1): 2962, 1627, 1260, 1016, 800, 691
1H NMR (CDCl3, ppm): 8.26–7.05 (aromatic, 18H), 4.07 (–OCH2, 2H), 2.59–0.89 (aliphatic, 19H).
Tg (DSC) = 160 °C
3 Result and discussion
3.1 Monomers and polymers synthesis
First, a pyridine-containing dibromo compound, 4(4-hydroxyphenyl)-2, 6-bis (4-bromophenyl) pyridine, (R1), was synthesized based on reaction of 4-bromo acetophenone and 4-hydroxy benzaldehyde according to a modified chichibabin reaction. Compound R1 was then converted to R2 and R3 by alkoxylation of its hydroxyl group with isopentyloxy and decyloxy groups by reaction with 1-boromo-3-methyl butane and 1-boromodecane, respectively (Scheme ).
Scheme 1 Monomers synthesis.
FTIR spectra of synthesized compound, R1, shows peak at 3151 cm−1 for hydroxyl group, and 821 cm−1 for C–Br. The compound did not show carbonyl peak and this is the difference with starting materials. The compound R1 was then converted to R2 and R3 by alkoxylation of its hydroxy group and its peak at 3151 for hydroxy group disappeared (Figure ). The 1H NMR and 13C NMR spectra of synthesized compounds also confirmed their synthesis (Figures ). The synthesized compounds, R1–R3 were also confirmed by CHN elemental analysis (experimental part).
Figure 1 Compared FTIR spectra of synthesized monomers.
Figure 2 1H NMR spectrum of R1.
Figure 3 13C NMR spectrum of R1.
Figure 4 1H NMR spectrum of R2.
Figure 5 13C NMR spectrum of R2.
Figure 6 1H NMR spectrum of R3.
Figure 7 13C NMR spectrum of R3.
Two polymers, P1 and P2 were synthesized by reaction of R1 and R2 with 2,2′-bithiophene-5,5′-dibronic acid bis(pinacol)ester according to Suzuki coupling in the presence of tetrakis (triphenyl phosphine)pd(0) as catalyst (Scheme ). FTIR spectra of both polymers showed aliphatic C–H, at about 2950 cm−1, a broad peak at about 1627 cm−1 for aromatic rings, a peak of about 1260 cm−1 for C–S, and at about 1010 cm−1 for C–O (Figure ). The structures of these polymers were also characterized using 1H NMR spectroscopy and the results confirmed the synthesis of polymers (Figures).
Scheme 2 Polymers synthesis.
Figure 8 Compared FTIR spectra of synthesized polymers.
Figure 9 1H NMR spectrum of P1.
Figure 10 1H NMR spectrum of P2.
The number average molecular weight (Mn) and weight average molecular weight (Mw) of these polymers were investigated using GPC and the results showed good chain growth for synthesized polymers (Table ). Because of shielding effect to both solvents of DMF and THF, the results of molecular weight for P2 could not be obtained.
Table 1. Molecular weight of synthesized polymers.
As an important factor, thermal stability of these polymers was studied using TGA. The polymers start to lose weight because of thermal degradation at around 200 °C. These weight losses are because of pendant alkoxy groups in polymer chains. For P2 with larger alkoxy groups, the weight loss is more than P1 and also the percent of remaining polymer at 500 °C is less (Figure ). The temperatures of 5% weight loss of these polymers were 260 and 230 °C, respectively, for P1 and P2. The thermal behavior of these polymers was investigated using DSC. The glass transition temperature (Tg) of these polymers were obtained and it was 190 °C for P1 and 160 °C for P2 (Figure ).
Figure 11 Compared TGA thermogram of synthesized polymers.
Figure 12 Compared DSC thermogram of synthesized polymers.
3.2 Photophysical properties
These two synthesized polymers have different properties. The ones with longer side chain (P2) are more electron donating than ones with smaller side chain (P1). So, the polymer P2 has lower band gap. The other effect is mobility of electrons on the polymers and here P1 is better than P2, because chains of P1 are nearer altogether and they can eliminate static charges. To calculate the amount of energy gap of the synthesized compounds, UV–vis spectroscopic method is used. The absorption spectra of the P1 and P2 in THF solution and solid state are shown in Figure . Based on Einstein formula, E = hc/λ, a conductive polymer, absorbs a photon when this photon energy is equal to electron transfer between orbitals. If this energy is less than band gap, the electron could not transfer from HOMO to LUMO. The λonset in UV–vis spectroscopy is an important factor for determining the band gap of compounds. Based on these data of λonset obtained in UV–vis spectra, and using the Einstein formula, the band gaps were measured and the results are summarized in Table .
Figure 13 Compared UV–vis spectra of synthesized polymers both in solution and solid state.
Table 2. Optical and electrochemical properties of synthesized compounds.
3.3 Electrochemical properties
The band gap, HOMO, and LUMO energy levels were estimated from the cyclic voltammetry (CV).
The cyclic voltammogram of polymers are shown in Figures. From the values of the onset oxidation potential (Eox, onset) and onset reduction potential (Ered, onset) of the compounds, the HOMO and LUMO energy levels as well as the electrochemical band gaps (Eg,EC) were calculated. In oxidation potential (Eox, onset), which occurred in positive potentials, polymer looses the electron which is related to HOMO and in reduction potentials (Ered, onset), the polymer obtains the electron related to LUMO. Therefore based on these data obtained in CV, using Figures, the band gap of the polymers were measured and the results are summarized in Table .
Figure 14 Cyclic voltammogram of P1.
Figure 15 Cyclic voltammogram of P2.
The HOMO and LUMO were calculated by these Equations Citation[50]:Here, E1/2, ferrocene = 0.488 eV.
The optical band gap calculated from UV–vis spectroscopy is smaller than the electrochemical band gap, which might be due to the interface barrier of charge injection.
4. Conclusion
In this research, two new polymers, P1 and P2, based on pyridine and thiophene derivatives were introduced. Based on our results, these polymers have low band gap and good solubility in organic solvents, which make them promising for use in active layers in solar cell applications.
Acknowledgments
The authors thank University of Isfahan providing facilities for this work and Central Laboratory of University of Isfahan for FTIR, CHN, NMR, and GPC analysis.
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