Structure–function relationships in conjugated materials containing tunable thieno[3,4-b]pyrazine units
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- Mulholland, M.E., Schwiderski, R.L. & Rasmussen, S.C. Polym. Bull. (2012) 69: 291. doi:10.1007/s00289-012-0718-x
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A series of 2,3-difunctionalized 5,7-bis(2-thienyl)thieno[3,4-b]pyrazines containing electron-donating and electron-withdrawing side chains are reported to evaluate the potential tuning effect of the side chains on the electronic properties of these common terthienyl building blocks. In order to further study the resulting effects of such side chains in polymeric materials, the dihexyloxy-functionalized terthienyl was copolymerized with fluorene and its electronic properties compared with a number of analogous materials.
KeywordsConjugated polymersThieno[3,4-b]pyrazineStructure–function relationshipsElectronic tuning
One of the critical parameters of conjugated organic polymers is the band gap (Eg), which is the energetic separation between the material’s filled valence and empty conduction bands [1–4]. As this corresponds to the HOMO–LUMO gap of the solid state material, the Eg determines such material properties as the lowest energy absorbance and the energy of any potential emission. Because of this, significant effort has been applied to developing methods for controlling the Eg of conjugated materials with the goal of producing technologically useful reduced band gap (Eg = 1.5–2.0 eV) and low band gap (Eg < 1.5 eV) polymers .
Materials and instrumentation
2,5-Bis(2-thienyl)-3,4-diaminothiophene (5) , 2,3-dibromothieno[3,4-b]pyrazine (6) , 2-(tributylstannyl)thiophene [20, 21], 2,3-dimethyl-5,7-bis(2-thienyl)thieno[3,4-b]pyrazine (7) [20, 21], and 2,7-dibromo-9,9-dioctyl-9H-fluorene (8)  were prepared as previously reported. THF was distilled from sodium benzophenone prior to use. CH2Cl2 was dried over CaH and distilled prior to use. DMF was dried over MgSO4 prior to use. Chromatographic separations were performed using standard column methods with silica gel (230–400 mesh). Basic silica gel was prepared by pretreating the silica with 3% Et3N in CH2Cl2. Unless otherwise stated, all other materials were reagent grade and used without further purification. All reactions were performed under a nitrogen atmosphere using oven-dried glassware. Melting points are corrected and were obtained using a heating block with a thermocouple connected to a digital thermometer. Unless otherwise stated, NMR spectra were obtained in CDCl3 on a 400 MHz spectrometer and referenced to the chloroform signal. Electrochemical measurements were performed on an EC Epsilon potentiostat using a Pt disc working electrode and a Pt wire counter electrode. Solutions consisted of 0.1 M TBAPF6 in CH3CN and were sparged with argon for 20 min prior to data collection and blanketed with argon during the experiment. Terthiophene samples were measured as millimolar solutions and polymeric samples were measured as solid-state films drop-cast onto the Pt disc working electrodes. All potentials are referenced to a Ag/Ag+ reference (0.1 M AgNO3/0.1 M TBAPF6 in CH3CN; 0.320 V vs. SCE) . UV–Visible spectra were measured on a dual-beam scanning spectrophotometer using samples prepared as dilute solutions in 1-cm quartz cuvettes or thin films spun onto glass slides.
Diamine 5 (1.39 g, 5.00 mmol) was added to absolute ethanol (80 mL), the mixture was heated with stirring until completely dissolved and allowed to cool to room temperature. 1,4-Dibromo-2,3-butanedione (1.89 g, 7.50 mmol) in 40 mL absolute ethanol was then added dropwise and the mixture was allowed to stir for 6 h. The mixture was then cooled to −25 °C, filtered, and washed with cold ethanol to give a purple solid (84–92% yield). 1H NMR: δ 7.66 (dd, J = 3.6 Hz, 1.2 Hz, 2H), 7.39 (dd, J = 5.2 Hz, 0.8 Hz, 2H), 7.11 (dd, J = 5.2 Hz, 3.6 Hz, 2H). 13C NMR: δ 150.1, 137.5, 134.1, 127.7, 127.4, 126.3, 125.7, 31.6. HRMS m/z 506.8290 [M+Na]+ (calcd for C16H10Br2N2NaO2S3 506.8265).
NaH (oil dispersion, 57% w/w, 0.40 g, 9.2 mmol) was washed with hexanes and then added to 1-hexanol (20 mL) and stirred overnight to ensure complete NaH consumption. The hexanol solution was then added dropwise to 9 (0.486 g, 1.00 mmol) in 50 mL dry CH2Cl2 and the mixture allowed to stir for 6 h. Saturated aqueous NH4Cl was then added, the CH2Cl2 removed by rotary evaporation, and the remaining aqueous hexanol solvent removed via vacuum distillation (45–50 °C at 15 mmHg). Water was then added and the mixture was extracted with CH2Cl2. The combined organic fractions were dried with MgSO4, concentrated and purified by silica chromatography (50% CH2Cl2/hexanes) to give a red–purple solid (50–60% yield). 1H NMR: δ 7.64 (dd, J = 4 Hz, 1.2 Hz, 2H), 7.36 (dd, J = 4.8 Hz, 1.2 Hz, 2H), 7.09 (dd, J = 4.8 Hz, 3.6 Hz, 2H), 4.86 (s, 4H), 3.62 (t, J = 6.8 Hz, 4H), 1.68 (p, J = 6.8 Hz, 4H), 1.39 (m, 4H), 1.30 (m, 8H), 0.89 (t, J = 2.8, 6H). 13C NMR: δ 152.4, 137.7, 134.6, 127.5, 126.7, 125.4, 125.0, 72.8, 71.6, 31.9, 30.0, 26.1, 22.8, 14.3. HRMS m/z 551.1846 [M+Na]+ (calcd for C28H36N2NaO2S3 551.1831).
Diethylamine (25 mL, 242 mmol) was added dropwise to 9 (0.243 g, 0.500 mmol) in dry CH2Cl2 (25 mL) and the mixture was heated at reflux for 5 h. Water was then added and the mixture was extracted with CH2Cl2 (3 × 50 mL). The combined organic fractions were washed with saturated aqueous NaHCO3 (2 × 100 mL) and brine (2 × 100 mL), dried with MgSO4, concentrated. The crude product was purified by basic silica chromatography (CH2Cl2) to give a dark red solid (60–98% yield). 1H NMR: δ 7.63 (dd, J = 3.6 Hz, 1.2 Hz, 2H), 7.36 (dd, J = 5.2 Hz, 1.2 Hz, 2H), 7.09 (dd, J = 5.2 Hz, 3.6 Hz, 2H), 4.10 (s, 4H), 2.73 (q, J = 7.2 Hz, 8H), 1.06 (t, J = 7.2 Hz 12H). 13C NMR: δ 154.6, 137.7, 135.0, 127.3, 126.5, 124.6, 124.5, 57.5, 46.6, 11.4. HRMS m/z 471.1707 [M+H]+ (calcd for C24H31N4S3 471.1705).
NaH (oil dispersion, 57% w/w, 1.00 g, 23.8 mmol) was washed with hexanes and then added to 60 mL DMF. 1-Hexanol (1.0 mL, 8.0 mmol) was added and the mixture stirred at room temperature for 5 min. Compound 6 (0.60 g, 2.0 mmol) was then added and the mixture was stirred for an additional 2 h. Aqueous NH4Cl was added and the mixture was extracted with CH2Cl2. The organic layer was dried with Na2SO4, concentrated, and purified by silica chromatography (97:3, hexane:diethyl ether), giving a white solid (90–95% yield). mp 69.9–71.1 °C. 1H NMR: δ 7.36 (d, J = 6.8 Hz, 2H), 4.41 (t, J = 6.8 Hz, 2H), 1.85 (p, J = 7.2 Hz, 2H), 1.37 (m, 6H), 0.91 (t, J = 2.4 Hz, 3H). 13C NMR: δ 150.2, 138.3 112.4, 67.2, 31.6, 28.6, 25.8, 22.7, 14.1.
A solution of 12 (0.38 g, 1.0 mmol) in 100 mL DMF was cooled to −78 °C. NBS (0.44 g, 2.5 mmol) was added, the reaction was warmed to −15 °C, and stirred for 3 h. The solution was poured into ice (100 mL) and stirred for 15 min to form a yellow solid. The solid was filtered, extracted in ether, and washed with 200 mL of water. The organic layer was collected, dried with MgSO4, concentrated, and purified by silica chromatography (95:5, hexane:diethyl ether) to give a yellow solid (67% yield). mp 45.2–46.4 °C. 1H NMR: δ 4.48 (t, J = 6.8 Hz, 2H), 1.86 (t, J = 6.8 Hz, 2H), 1.38 (m, 7H), 0.89 (t, J = 2.4 Hz, 3H); 13C NMR: δ 151.1, 136.3, 98.8, 67.9, 31.6, 28.5, 25.8, 22.7, 14.1.
2-(Tributylstannyl)thiophene (0.39 g, 1.08 mmol) and 13 (0.21 g, 0.42 mmol) were combined in a 250 mL flask, evacuated, and backfilled with N2. Dry THF (100 mL) was then added by syringe, followed by a second N2 cycling. PdCl2(PPh3)2 (0.015 g, 0.02 mmol) was added, the solution heated to reflux, and stirred for 16 h. The solution was then allowed to cool to room temperature and solvent removed via rotary evaporation. The resulting material was dissolved in CH2Cl2, washed with H2O, dried with MgSO4, and concentrated via rotary evaporation. Purification was done by silica chromatography in (95:5, hexanes:CH2Cl2) yielding an orange solid (48% yield). 1H NMR: δ 7.45 (dd, J = 3.0, 1.2 Hz, 2H), 7.31 (dd, J = 4.5,1.2 Hz, 2H), 7.06 (dd, J = 3.0,4.5 Hz, 2H), 4.55 (t, J = 7.0 Hz, 4H), 1.93 (t, J = 7.0 Hz, 4H), 1.52–1.38 (m, 12H), 0.94 (t, J = 6.5 Hz, 6H). 13C NMR: δ 150.16, 135.40, 134.03, 127.04, 125.47, 123.29, 120.48, 68.08, 31.77, 28.52, 25.95, 22.80, 14.22.
Dry hexanes (60 mL) was added via syringe to a flask containing 14 (0.06 g, 0.12 mmol) and the solution was cooled to 0 °C. TMEDA (0.05 mL, 0.35 mmol) was then added followed by BuLi (0.13 mL, 0.33 mmol) and the solution was stirred for 2 h. Me3SnCl (0.3 mL, 0.33 mmol) was syringed into the solution and the reaction was allowed to stir overnight. The solution was poured over Et3N-treated silica gel, filtered, and rinsed with 100 mL hexanes. The solution was concentrated via rotary evaporation to yield a yellow liquid (99%). 1H NMR: δ 7.44 (d, J = 3.2, 2 H), 7.099 (d, J = 1.6, 2H), 4.40 (t, J = 6.4, 4H), 1.81 (m, J = 6.8, 4H), 1.53–1.20 (m, 16 H), 0.86 (t, J = 6.8, 6 H), 0.40 (s, 18H).
Fluorene 8 (0.06 g, 0.12 mmol), 15 (0.10 g, 0.12 mmol), Pd2dba3 (0.002 g, 0.002 mmol), and P(o-tolyl)3 (0.02 g, 0.08 mmol) were combined in a flask. N2-purged toluene (15 mL) was then added by syringe and the solution evacuated and backfilled with N2. The reaction was then placed in an oil bath (95 °C) and allowed to react for 4 days. The reaction was then cooled, poured into 300 mL methanol, and filtered. The soluble fraction of polymer was collected in CHCl3 and isolated by rotary evaporation. Further purification was accomplished by further washes with MeOH yielding a red solid (75%). 1H NMR: δ 7.50 (m), 7.44 (m), 7.30 (m), 4.55 (br t), 1.92 (m), 1.6-0.6 (m). GPC: Mw = 4000, Mn = 2700, PDI = 1.48.
Results and discussion
The most prolific class of TP-based oligomers consists of terthienyls containing a central TP unit, the first examples of which were reported by Yamashita and co-workers in 1994 . Such terthienyls are common precursors to TP-based copolymeric materials, resulting in a wide variety of materials with reported band gaps of 1.17–1.78 eV . Many of these materials have also been utilized as donor materials for bulk heterojunction organic photovoltaic (OPV) devices.
While the production of terthienyls from 9 is simple and straightforward, the α-methylene of the side chains reduces the potential electronic influence on the conjugated unit and the direct coupling of electron-donating or electron-withdrawing groups to the TP unit should result in a greater amount of electronic modulation. To accomplish the production of such terthienyls, substitution of dibromothieno[3,4-b]pyrazine 6 was accomplished as previously described  to generate the desired functionalized TP 12, as shown in Fig. 3. Treatment with NBS then allowed simple bromination of the thiophene α-positions, followed by Stille coupling with stannylthiophene to generate the desired terthienyl 14.
Optical and electrochemical data for theino[3,4-b]pyrazine-based terthienyls
Epa (V, oxidation)b
E½ (V, reduction)b
Electrochemical characterization of the terthienyls by cyclic voltammetry (CV) allows a more detailed study of the electronic effects on the respective HOMO and LUMO levels. Typical of monomeric TPs, the TP-based terthienyls exhibit a well-defined irreversible oxidation assigned to the oxidation of the terthiophene backbone, as well as a quasireversible pyrazine-based reduction. The measured electrochemical potentials are provided in Table 1. As previously seen for monomeric TPs , the potential of oxidation becomes more positive as the electron-withdrawing nature of the side chain is increased, indicative of a stabilization of the HOMO. An important difference, however, is that this effect is significantly attenuated in the terthienyl series. Thus, the change in potential of oxidation from the hexyloxy to methylbromo functional groups is nearly 300 mV in the monomeric TPs , but only 170 mV in the terthienyl series is reported here. This difference is due to the fact that the effect of the two additional 2-thienyl groups in the terthienyls contributes more significantly to the HOMO, resulting in a much greater effect on the HOMO energy, as illustrated by a shift in the potential of oxidation by 850 mV from monomeric TP to terthienyl [7, 9]. Thus, the destabilization of the HOMO by the added external thiophenes effectively overpowers the weaker contribution of the TP side chains.
In contrast, the addition of the external thiophenes contribute much less to the terthienyl LUMO and only results in a slight stabilization of the LUMO levels as illustrated by the slightly lower reduction potentials of the terthienyls in comparison to the monomeric TP analogues . As the pyrazine contributes most significantly to the LUMO, the TP functional groups provide the greatest effect on the LUMO energies, as illustrated by the nearly 800 mV shift in the corresponding reduction potentials. Thus, the large red shifts in the energy of the CT transitions discussed above are primarily due to stabilization of the LUMO energy levels through the use of electron-withdrawing side chains.
Interestingly, Helgesen and Krebs  reported polymer 4b that incorporates phenylester side chains on the TP unit. While these phenylesters were applied as thermocleavable side chains and not as a means to tune the polymer electronic properties, the significantly electron-withdrawing nature of the side chains resulted in a reduction of the band gap. As a consequence of the side chain applied, a polymeric framework normally resulting in reduced band gap materials can generate a true low band gap system with an Eg of 1.37 eV. While the electrochemistry was not reported, it would be expected from the trends above that this also resulted in stabilization of both the HOMO and the LUMO of the material.
The results above illustrate that the ability to incorporate either electron-withdrawing or electron-donating groups into the 2- and 3-positions of TPs allows the direct tuning of the electronic properties of TP-based materials. In the application to copolymeric materials, the effect of the side chains on the HOMO is diminished, but largely determines the LUMO energy and as a result the material’s band gap as well. The ability to modulate the properties of materials via TP functional groups would allow one to forgo the more complex tuning approaches of recent efforts and provide means for the production of systems that are much more structurally and synthetically simple. This would also provide the potential to achieve the desired electronic and optical properties for various device applications while holding the backbone structure constant and thus resulting in reduced morphological changes from material to material.
The authors wish to thank the National Science Foundation (DMR-0907043) and North Dakota State University for support of this research.