Organic Electronic Devices Week 1: Semiconductor Synthesis and Characterization Lecture 1.2: Synthesis of Poly(3-alkylthiophenes) (P3ATs) Bryan W. Boudouris Chemical Engineering Purdue University 1
Lecture Overview and Learning Objectives Concepts to be Covered in this Lecture Segment Chemical Syntheses of Poly(3-alkylthiophenes) (P3ATs) Grignard Metathesis (GRIM) Reaction Mechanism Regioregularity of Semiconducting Polymers Learning Objectives By the Conclusion of this Presentation, You Should be Able to: 1. Recall and transcribe three reaction mechanisms by which poly(3- alkylthiophenes) (P3ATs) can be synthesized. 2. Explain what is meant by regioregularity within a semiconducting polymer and distinguish between head-to-tail and head-to-head configurations in P3ATs. Furthermore, be able to predict which type of configuration will lead to better charge transport in P3AT semiconductors. 3. Name at least three reasons why the Grignard metathesis (GRIM) method is useful in the synthesis of semiconducting polymers.
P3ATs are an Important Class of Organic Electronic Materials Highlights of P3ATs Well-understood synthetic reaction mechanisms Side Chain Affects Melting Temperature Many synthetic schemes allowed for the controlled polymerization of P3ATs The thermal and structural properties of these materials is well-studied and fairly wellunderstood One of the most heavilyimplemented classes of polymers in organic field-effect transistors (OFETs) and organic photovoltaic (OPV) devices Platform material to elucidate many common organic electronic device concepts Further Reading: Ho, V.; Boudouris, B. W.; Segalman, R. A. Macromolecules 2010, 43, 7895.
P3ATs Can Be Synthesized Using Oxidative Polymerizations Electrochemical Oxidation of 3ATs to form P3ATs Chemical Oxidation of 3ATs to form P3ATs Both reactions proceed via a radical cation-based polymerization mechanism. Reaction Mechanism Notes Monomer does not require any specific substitution at either the 2 or the 5 position (PRO). Very simple chemistry (PRO). Because this is a radical-mediated polymerization, there is little control over how the R groups are arranged relative to one another. Furthermore, it is difficult to control the end groups of the polymer chain (CONS). For the chemical oxidation polymerization, residual metal catalyst can disrupt device operation (CON). Further Reading: McCullough, R. D. Adv. Mater. 1999, 2, 93.
P3ATs Can Be Synthesized Using the Rieke Method Rieke Synthetic Scheme Ni(dppe)Cl 2 = [1,2-bis-(diphenylphosphino) ethane] nickel(ii) chloride is the catalyst. The nickel catalyst allows for the insertion of the nickel species into the bonds of the activated thiophene manners simultaneously. The polymerization reaction proceeds by the coupling of the two thiophene units and the recovery of the Ni(dpppe)Cl 2 species. The chemistry of the ligands effects the final polymerization product. Further Reading: Chen, T.-A.; Xu, X.; Rieke R. D. J. Am. Chem. Soc. 1995, 117, 233.
P3ATs Can Be Synthesized Using the Rieke Method Rieke Synthetic Scheme Major Product Minor Product Reaction Mechanism Notes The reaction mechanism allows for the synthesis of high molecular weight, low dispersity semiconducting polymers (PRO); however, it does require air-free reaction conditions (CON). The nickel catalyst can be removed from the final polymer product easily (PRO). However, the Zn* reagent is more difficult to remove (CON). Bromination of the thiophene monomers is required (CON). The cryogenic temperatures of the first step of the reaction make scaling the reaction to industrial scales more difficult (CON). Further Reading: Chen, T.-A.; Xu, X.; Rieke R. D. J. Am. Chem. Soc. 1995, 117, 233.
P3ATs Can Be Synthesized Using the Grignard Metathesis Method Grignard Metathesis (GRIM) Synthetic Scheme Minor Product Major Product Ni(dppp)Cl 2 = [1,2-bis-(diphenylphosphino) propane] nickel(ii) chloride is the catalyst. Just like in the Rieke synthetic method, the nickel catalyst allows for the insertion of the nickel species into the bonds of the activated thiophene manners simultaneously. The exact mechanism of this insertion and reductive elimination reaction will be detailed in just two slides. Further Reading: Osaka, I.; McCullough, R. D. Acc. Chem. Res. 2008, 41, 1202.
P3ATs Can Be Synthesized Using the Grignard Metathesis Method Grignard Metathesis (GRIM) Synthetic Scheme Major Product Minor Product Reaction Mechanism Notes The reaction mechanism allows for the synthesis of high molecular weight, low dispersity semiconducting polymers (PRO); however, it does require air-free reaction conditions (CON). The nickel catalyst and the Grignard reagent both can be removed from the final polymer product easily (PRO). Furthermore, the reaction temperatures are readily-compatible with largescale syntheses (PRO). Bromination of the thiophene monomers is required (CON). The reaction mechanism allows for end group control and, thus, unique architectures (PRO). Further Reading: Osaka, I.; McCullough, R. D. Acc. Chem. Res. 2008, 41, 1202.
Mechanism of the GRIM Polymerization Method Reinsertion of the nickel catalyst Mechanism necessitates at least 1 tail-to-tail linkage Further Reading: Stefan, M. C., Javier, A. E.; Osaka, I.; McCullough, R. D. Macromolecules 2008, 42, 30.
Regioregularity in P3AT Semiconductors is Important Each Dyad (i.e., 2 Consecutive Repeat Units) Can Have Different Linkages Head-to-Tail (or Tail-to-Head) Head-to-Head Tail-to-Tail Head-to-Tail Linkages Lead to More Crystalline Materials (In General) The degree of regioregularity is measured through the number of head-to-tail linkages in the polymer. The regular back-and-forth arrangement of the side chains allows for the polymer to adopt a regular packing motif in a relatively straightforward manner. If a molecule has all head-to-tail linkages, it is said to be 100% regioregular. If a molecule has 25% head-to-tail, 25% tail-to-head, 25% head-to-head, and 25% tail-to-tail, it is said to be regiorandom or regioirregular.
Beyond Simple P3ATs Extension of the GRIM Method Controlling the chemistry of the P3AT block polymers allowed for the control of the nanostructure. All scale bars represent 200 nm. Further Reading: Ho, V.; Boudouris, B. W.; et al. J. Am. Chem. Soc. 2011, 133, 9270.
Positive Aspects and Drawbacks of P3AT Synthetic Schemes Reaction Type Benefits Limitations Oxidative Polymerization Method Rieke Polymerization Method GRIM Polymerization Method 1) Requires straightforward experimental equipment and scales easily 2) Can produce regiorandom P3ATs 3) Useful if you want to grow P3ATs from the surface of a metal electrode 1) Can generate polymers with controlled molecular weight and dispersity 2) Can vary the regioregularity by changing the design of the catalyst 1) Can generate polymers with controlled molecular weight and dispersity 2) Can create highly regioregular polymers 3) Easy to purify the final polymer product 4) Provides control over the polymer end group 1) Cannot generate completely regioregular P3ATs 2) There is little control over the molecular weight or dispersity 3) There is no control over the polymer end group 4) It is difficult to purify the final polymer 1) Provides no control over the end group of the polymer 2) Requires an air-free environment and the Zn* is very unstable in ambient conditions 3) It is difficult to remove the Zn* form the final polymer 1) Requires an air-free environment and the Grignard reagent is very unstable in ambient conditions 2) Difficult to tune the regioregularity of the polymers
Summary and Preview of the Next Lecture Poly(3-alkylthiophenes) (P3ATs) can be synthesized in a number of different manners. These include the oxidative polymerization of 2,5-unsubstituted thiophene moieties using either chemical or electrochemical means. Furthermore, 2,5-dibrominated 3- alkylthiophene monomers can be polymerized through the use of the Rieke or Grignard metathesis (GRIM) methodologies. Each of these strategies have pros and cons, and the user must determine which procedure is of the most utility for his/her particular process. The GRIM method, initially developed by Richard McCullough and co-workers, is one of the more powerful routes by which to generate P3ATs. This is because the GRIM method allows for the synthesis of high molecular weight, low dispersity P3ATs whose polymer chain length and macromolecular architecture can be changed through simple changes to the chemistry. This has allowed for synthesis of advanced P3AT macromolecular architectures using the GRIM method. As such, the synthesis of end-functionalized P3ATs and P3AT-based block polymers occurs readily in the current state-of-the-art. Next Time: Synthetic Protocols for Emerging Polymer Semiconductors