In the Laboratory
Regiochemistry of Poly(3-hexylthiophene): Synthesis and Investigation of a Conducting Polymer Ted M. Pappenfus,* David L. Hermanson, Stuart G. Kohl, Jacob H. Melby, Laura M. Thoma, and Nancy E. Carpenter Division of Science and Mathematics, University of Minnesota, Morris, Minnesota 56267 *
[email protected] Demetrio A. da Silva Filho and Jean-Luc Bredas School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400
The field of conducting polymers continues to grow based on the widespread application of these conjugated materials for use in important areas related to energy, electronics, and the environment (1). The discovery in the late 1970s that conjugated polymers can be made highly electrically conductive upon oxidation or reduction (2) has opened up new possibilities for materials combining the unique optical, electronic, and mechanical properties of plastics with those of inorganic materials. Although organic materials, such as conducting polymers, may compete in some applications with conventional inorganic electronic materials, in most instances they open the way to a new range of applications where inorganics cannot be exploited. This is due in large part to the low density, flexibility, and rollto-roll processing properties of plastics (3). Although excellent undergraduate laboratory experiments outlining the preparation of conducting polymers have been reported in this Journal (4) and others (5), these experiments have focused on the thin-film electropolymerization of these materials. In contrast, experiments involving the bulk synthesis of a conducting polymer have not yet been reported. The synthesis of a bulk material allows students to take full advantage of the rich chemistry and applications of this class of materials. Toward this end, we have developed a series of experiments based on the synthesis and regiochemistry of poly(3-hexylthiophene) (P3HT), a workhorse in the field of conducting polymers. We note that the synthetic procedures reported here lead to polymers in their neutral (nonoxidized), insulating form. As is evident from its structure, 3-hexylthiophene is not a symmetric molecule. As the monomer polymerizes to form the polymer, three unique couplings are possible (Figure 1). Polymers that contain a mixture of these couplings are referred to as regiorandom (or irregular). Head-to-head (HH) couplings in regiorandom poly(3-alkylthiophenes) (P3ATs) cause thiophene rings to twist owing to steric interactions of the alkyl groups. This twisting of rings results in decreased conjugation of the polymer (6). On the other hand, polymers that contain only head-to-tail (HT) couplings are referred to as regioregular. This regioisomer can adopt a more planar and highly conjugated backbone owing to decreased steric interactions of the alkyl groups. The resulting decrease of the torsion angles between thiophene rings in regioregular P3AT leads to efficient inter- and intrachain conductivity 522
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pathways that are not present in the regiorandom isomer. For example, regiorandom forms of P3ATs reach upon oxidation electrical conductivities from 0.1 to 10 S/cm, whereas the regioregular forms have values from 150 to 1000 S/cm (6, 7). Because of their relative ease of synthesis, regiorandom forms of P3ATs were reported first, the most common synthetic method being oxidative polymerization using FeCl3 (7). At about the same time in the early 1990s, two independent reports surfaced outlining the synthesis of regioregular P3ATs: Rieke et al. reporting the use of organozinc reagents (8) and McCullough et al. reporting the use of organomagnesium reagents (9). Although both of these methods provide reasonable strategies for the preparation of P3ATs, they are not well suited for adaptation in the undergraduate lab owing to significant drawbacks (e.g., the need for highly purified starting materials, cryogenic reaction temperatures, and long reaction times). Fortunately, the McCullough group later reported (10) a more practical, efficient, and affordable method for the preparation of P3ATs. It is this method that we have utilized in our undergraduate laboratories for the preparation of P3HT. Experimental Overview Working in pairs, students prepare P3HT over the course of two lab periods. In the first lab period, each student prepares either the monomer 2,5-dibromo-3-hexylthiophene or the nickel catalyst Ni(dppp)Cl2, where dppp is bis(diphenylphosphino)propane, used
Figure 1. Pairwise coupling of the monomer unit (top) to form the regiochemical isomers (bottom).
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In the Laboratory
Scheme 1. Synthesis of the Monomer (Top) and Nickel Catalyst (Bottom) Used for the Preparation of P3HTa
a
NBS = N-bromosuccinimide.
Scheme 2. Synthesis of P3HTa
a
Figure 2. 1H NMR spectra of regioregular P3HT (top) prepared by the GRIM method and regiorandom P3HT (bottom) prepared by the FeCl3 method.
Typical student yields were 45% regioregular polymer.
for the polymerization (Scheme 1). The syntheses of each of these materials are adapted from slight modifications of previously reported procedures (10, 11). Both can be prepared safely and easily from commercially available reagents. The monomer is prepared by the addition of 2.0 g of N-bromosuccinimide (NBS) to a solution of 0.90 g of 3-hexylthiophene in THF (6 mL). After 1.5 h, the THF is removed and hexanes are added to suspend the succinimide, which is subsequently filtered. The resulting solution is further purified by passing through a micro silica gel column. Typical student yields for the preparation of the monomer are near 1.4 g (80%). The nickel catalyst is prepared on the microscale as previously described (11) by the addition of 151 mg of 1, 3-bis(diphenylphosphino)propane to 100 mg of nickel(II) chloride hexahydrate. Typical student yields for the preparation of the nickel catalyst are 127 mg (64%). Students characterize the dibromomonomer by GC-MS and 1H and 13C NMR. They are asked to compare their values to those found in the literature (12). Typically, a small quantity (98% and 73% for the regioregular and regiorandom polymers, respectively. End group analysis of 1H NMR spectra can also be used to calculate the number average molecular weight (Mn) of P3HT. This method is well described in the literature using simple integration methods (16). Typical results from the studentprepared P3HT lead to degrees of polymerization from 50-65 monomer units corresponding to Mn = 8300-10,800. Advanced methods of molecular weight determination such as gel permeation chromatography were not utilized in this experiment but are appropriate for more advanced courses. UV-vis spectroscopy can also be used to examine the regiochemistry of P3ATs based on energies of the bands in the absorption and emission spectra. For example, chloroform solutions of student-prepared regioregular P3HT exhibit a λmax value near 450 nm, whereas regiorandom P3HT exhibits a blue-shifted λmax near 435 nm. The energy of the regiorandom peak is largely dictated by the percentage of HT-HT couplings in the polymer (12). Smaller shifts are observed in the emission spectra. This has been attributed to greater relief of conformational strain in the excited state (17). More dramatic, however, are the differences seen in the thin-film absorption spectra of these polymers (Figure 3). Regiorandom P3HT exhibits a broad absorption spectrum with a λmax near 475 nm whereas regioregular P3HT films are red-shifted (λmax = 557 nm) and show fine structure. These values are in excellent agreement with those reported previously (18). The shifts seen in the absorption spectra can be attributed to differences in energy of the π-π* transitions and are directly related to the extent of conjugation in these materials. In comparison to regiorandom P3HT, regioregular P3HT exhibits lower-energy electronic transitions resulting from greater orbital overlap from the increased 524
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Figure 4. HOMO-LUMO gap versus the reciprocal of the polymer length (indicated by the number of oligomer units) for regioregular 3-methylthiophene.
HT-HT couplings present in the polymer. Furthermore, the shifts in the solid-state absorption spectra also illustrate the ability of the regioregular P3HT to form more ordered solid-state structures with reduced band gaps. In this experiment, students use density functional theory (DFT) calculations to understand the experimental data of the polymers. Because of their computational efficiency and accuracy, DFT calculations have become one of the most popular and versatile quantum mechanical methods utilized by chemists (19). Not surprisingly, chemical applications of DFT have been extended to undergraduate chemistry laboratories (20). A common approach to finding the theoretical band gap of a conjugated polymer is to extrapolate the linear curve of calculated HOMO-LUMO gaps, ΔE, of a series of oligomers as a function of the reciprocal of the number of monomer units, 1/n (21). In this investigation, students work cooperatively to calculate HOMO-LUMO gaps (determined at the B3LYP/6-31G (d) level) for a series of regioregular 3-methylthiophene oligomers (substitution of methyl for hexyl is performed to save computational time and has only modest impact on the results). The resulting plot of ΔE as a function of 1/n provides an extrapolated value of 1.78 eV (Figure 4). This is in excellent agreement with the experimental band gap values (1.7-1.8 eV) of regioregular P3HT (18). DFT calculations are also used in this experiment to compare experimental and theoretical infrared data of the 3-hexylthiophene monomer as well as examining the conformational and electronic effects of the regiochemical triads of P3HT. The latter exercise illustrates the effect of HH couplings on intramolecular ring torsions of the thiophene rings, which reduces π overlap and increases separation of HOMO-LUMO energy levels. Details of all DFT calculations are found in the supporting material. Summary These experiments have been tested and can be performed with confidence by undergraduate chemistry students. All of the experiments can be accomplished with commercially available reagents and standard glassware and equipment. With the exception of the 3-hexylthiophene starting material, the other materials used in this experiment are fairly inexpensive. We have successfully performed these experiments over three lab periods in our second-year chemistry laboratory course, although a two lab-period experiment is plausible if the activities are limited to the synthesis of P3HT. Alternatively, these experiments are equally
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In the Laboratory
suitable for an upper-level polymer, organic synthesis, or inorganic laboratory course. Regardless of the specific implementation, however, the experiments outlined here illustrate the rich chemistry of P3ATs. Additional efforts are currently underway to extend the chemistry of P3ATs (e.g., molecular weight determination, thermal analysis, X-ray analysis) and other conducting polymers across the undergraduate chemistry curriculum. Acknowledgment We thank the National Science Foundation (Division of Undergraduate Education through Grant No. DUE-0535763 and the MRSEC program through awards DMR-0212302 and DMR-0819885) and the University of Minnesota, Morris (UMM) for financial support. T.M.P. acknowledges UMM Faculty Research Enhancement Funds supported by the University of Minnesota Office of the Vice President for Research. We also thank Megan L. Mekoli and UMM undergraduates for conducting the experiments and Seth C. Rasmussen and Paul C. Ewbank for helpful suggestions and insight. Literature Cited 1. Handbook of Conducting Polymers, 3rd ed.; Skotheim, T. A., Reynolds, J. R., Eds.; CRC Press: New York, 2007. 2. Chiang, C. K.; Fincher, C. R., Jr.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.; Louis, E. J.; Gau, S. C.; MacDiarmid, A. G. Phys. Rev. Lett. 1977, 39, 1098–1101. 3. (a) Facchetti, A. Mater. Today 2007, 10, 28–37.(b) Organic Electronics: Materials, Manufacturing and Applications; Klauk, H., Ed.; Wiley-VCH: Weinheim, Germany, 2006. 4. (a) Ramanaviciene, A.; Finkelsteinas, A.; Ramanavicius, A. J. Chem. Educ. 2006, 83, 1212–1214. (b) Cortes, M. T.; Moreno, J. C. J. Chem. Educ. 2005, 82, 1372–1373. (c) Bendikov, T. A.; Harmon, T. C. J. Chem. Educ. 2005, 82, 439–441. (d) Sadik, O. A.; Brenda, S.; Joasil, P.; Lord, J. J. Chem. Educ. 1999, 76, 967–970. (e) Bunting, R. K.; Swarat, K.; Yan, D.; Finello, D. J. Chem. Educ. 1997, 74, 421–422. (f) Sherman, B. C.; Euler, W. B.; Force, R. R. J. Chem. Educ. 1994, 71, A94–A96. 5. Lunsford, S. K.; Zhang, H. Chem. Educ. 2004, 10, 10–14. 6. Jeffries-El, M.; McCullough, R. D. In Handbook of Conducting Polymers, 3rd ed.; Skotheim, T. A., Reynolds, J. R., Eds.; CRC Press: New York, 2007; Vol. 1, pp 9/1-9/49.
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7. McCullough, R. D. Adv. Mater. 1998, 10, 93–116. 8. Chen, T.-A.; Rieke, R. D. J. Am. Chem. Soc. 1992, 114, 10087– 10088. 9. McCullough, R. D.; Lowe, R. D. J. Chem. Soc., Chem. Commun. 1992, 70–72. 10. Loewe, R. S.; Khersonsky, S. M.; McCullough, R. D. Adv. Mater. 1999, 11, 250–253. 11. (a) Szafran, Z.; Pike, R. M.; Singh, M. M. Microscale Inorganic Chemistry, A Comprehensive Laboratory Experience; John Wiley & Sons: New York, 1991. (b) Van Hecke, G. R.; Horrocks, W. D., Jr. Inorg. Chem. 1966, 5, 1968–1974. 12. Chen, T.-A.; Wu, X.; Rieke, R. D. J. Am. Chem. Soc. 1995, 117, 233–244. 13. Loewe, R. S.; Ewbank, P. C.; Liu, J.; Zhai, L.; McCullough, R. D. Macromolecules 2001, 34, 4324–4333. 14. Amou, S.; Haba, O.; Shirato, K.; Hayakawa, T.; Euda, M.; Takeuchi, K.; Asai, M. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 1943–1948. 15. Barbarella, G.; Bongini, A.; Zambianchi, M. Macromolecules 1994, 27, 3039–3045. 16. Iovu, M. C.; Sheina, E. E.; Gil, R. R.; McCullough, R. D. Macromolecules 2005, 38, 8649–8656. 17. Xu, B.; Holdcroft, S. Macromolecules 1993, 26, 4457–4460. 18. McCullough, R. D.; Ewbank, P. C. In Handbook of Conducting Polymers, 2nd ed.; Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Marcel Dekker: New York, 1998; pp 225-258. 19. Koch, W.; Holthausen, M. C. A Chemist's Guide to Density Functional Theory, 2nd ed.; Wiley-VCH: New York, 2000. 20. For example, see the following and references therein: (a) Bell, S.; Dines, T. J.; Chowdhry, B. Z.; Withnall, R. J. Chem. Educ. 2007, 84, 1364–1370. (b) Autschbach, J.; Le Guennic, B. J. Chem. Educ. 2007, 84, 156–171. (c) Waas, J. R. J. Chem. Educ. 2006, 83, 1017–1021. (d) Streit, B. R.; Geiger, D. K. J. Chem. Educ. 2005, 82, 111–115. 21. The following reference provides insight in determining band gaps of conjugated polymers: Zade, S. S.; Bendikov, M. Org. Lett. 2006, 8, 5243–5246.
Supporting Information Available Student handouts for the experiments; instructor notes; experimental procedures; and relevant spectra. This material is available via the Internet at http://pubs.acs.org.
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