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Perspective
Conjugated Polymers Inspired by Crystalline Silicon Eric A. Marro, and Rebekka S. Klausen Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00131 • Publication Date (Web): 08 Mar 2019 Downloaded from http://pubs.acs.org on March 9, 2019
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Chemistry of Materials
Conjugated Polymers Inspired by Crystalline Silicon Eric A. Marro, Rebekka S. Klausen* Department of Chemistry, Johns Hopkins University, 3400 N. Charles St, Baltimore, MD 21218 ABSTRACT: Modern technologies depend on the semiconductor silicon. Silicon nanostructures evince compelling properties including luminescence and biodegradability. Silicon-based soft matter is less explored, despite the attractive properties of conjugated polymers and small molecules inspired by crystalline silicon. This perspective describes the major synthetic approaches to polysilanes and summarizes innovations in the preparation of structurally complex functional polysilanes that combine unusual conjugation phenomena, solution processability, and morphological control. A connection is drawn between the limitations of current polymerization methodology and fine control of structure-property relationships.
Bulk silicon is a versatile material underpinning contemporary technologies, such as computer chips and the silicon-based solar panels that dominate the commercial market. With respect to emerging technologies, the large surface area of silicon nanowires and silicon’s exceptional charge capacity is exploited in lithium ion batteries. While bulk silicon has an indirect band gap and is not luminescent, Canham’s discovery of light emission from porous silicon (p-Si)1,2 triggered intense interest in silicon for optoelectronic applications as quantum confinement introduces a tunable band gap.3,4 Silicon nanowires are also intensely investigated as photoelectrochemical materials for conversion of solar energy into clean fuels.5,6 Silicon’s abundance (silica, SiO2, is the major component of the earth’s crust)7 contributes to its widespread application, as earth abundance ensures that silicon is both a relatively affordable and sustainable choice for long-term applications. Silicon nanocrystals appeal relative to Group II-VI and III-V quantum dots for their earth abundance. Colloidal silicon nanocrystals, their polymer composites, and covalently integrated polymer-nanocrystal hybrids were recently reviewed.8 In addition to its economic advantages, silicon nanomaterials appeal for biomedical applications due to their biodegradability and low toxicity.9–12
property relationships. Our work14–16 on building blocks mimicking the crystalline silicon lattice is described, as well as efforts by Fujiki, Koe, and others17,18 on integrating functional organic side chains into polysilanes. Further expansion of the scope of functional polysilanes depends on creative synthetic strategies, such as those highlighted herein, as well as the development of new and milder silane polymerization methods. Overview of Synthetic Methods. The design of functional silicon-based polymers is limited by the small number of suitable building blocks and synthetic methods.13,19 Three synthetic approaches – reductive, dehydrocoupling, and anionic ring-opening polymerization – are most commonly used, each with their own restrictions in scope and efficiency.
These attractive qualities motivate the exploration of well-defined conjugated polymers containing Si–Si units (polysilanes).13 Polysilanes combine unusual electronic phenomena (e.g. -conjugation) arising from their inorganic backbones with the desirable physical properties of organic materials like solution processability. Polysilanes exhibit lower energy optical transitions than analogous alkanes, with typical onsets of absorption in the near-UV (300–400 nm). This Perspective reviews advances in polysilane structural complexity, with a focus on synthesis-structure-
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Figure 1. Polysilane overview. Structural evolution and building blocks & synthesis.
With a century-long history,20,21 the most widely used approach to high molecular weight polysilanes is alkali metal-mediated reductive Wurtz coupling of dihalodialkylsilanes. The mechanistic complexity of the heterogeneous reaction and the strongly reducing conditions (typically sodium in refluxing toluene) limit the complexity of suitable precursors to simple alkyl and aryl side chains: chloro- and bromoarenes, multiple bonds, and ether groups directly attached to silicon are not tolerated.19 Polysilanes synthesized by Wurtz-coupling include polydimethylsilane (poly(SiMe2), poly(di-n-hexylsilane) (PDHS, poly(Si(n-C6H13)2), and polymethylphenylsilane (PMPS), poly(SiMePh). Insights from Jones et al. led to milder room temperature conditions.22 Other reductive polymerizations are known, including chemical (Sm/SmI2)23 and electrochemical reductions.24–26 Transition metal-catalyzed dehydrocoupling polymerization of hydrosilanes (e.g. phenylsilane) emerged in the 1980’s.27 While functional groups like aryl bromides were tolerated,28 the typical catalysts, group 4 metallocenes, were exquisitely sensitive to steric effects and until recently only monosubstituted silanes gave reasonable degrees of polymerization. Even these monomers typically yielded modest molecular weight polymers (
