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Related Concept Videos

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Electrocyclic reactions are reversible reactions. They involve an intramolecular cyclization or ring-opening of a conjugated polyene. Shown below are two examples of electrocyclic reactions. In the first reaction, the formation of the cyclic product is favored. In contrast, in the second reaction, ring-opening is favored due to the high ring strain associated with cyclobutene formation.
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The absorption of UV–visible light by conjugated systems causes the promotion of an electron from the ground state to the excited state. Consequently, photochemical electrocyclic reactions proceed via the excited-state HOMO rather than the ground-state HOMO. Since the ground- and excited-state HOMOs have different symmetries, the stereochemical outcome of electrocyclic reactions depends on the mode of activation; i.e., thermal or photochemical.
Selection Rules: Photochemical Activation
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Carrier generation is the process by which electron-hole pairs (EHPs) are created within the semiconductor. In direct-bandgap semiconductors, such as gallium arsenide (GaAs), this occurs efficiently when energy absorption prompts valence electrons to leap into the conduction band, leaving behind holes.
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The stereochemistry of electrocyclic reactions is strongly influenced by the orbital symmetry of the polyene HOMO. Under thermal conditions, the reaction proceeds via the ground-state HOMO.
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The cationic polymerization mechanism consists of three steps: initiation, propagation, and termination. In the initiation step of the polymerization process, the π bond of a monomer gets protonated by the Lewis acid catalyst, which is formed from boron trifluoride and water. The protonation of the π bond generates a carbocation stabilized by the electron‐donating group. In the propagation step, the π bond of the second monomer acts as a nucleophile and attacks the...
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The skeletal structure of polymers synthesized via radical polymerization is always branched. For example, the polymerization of ethylene by radical polymerization results in a low-density grade of polyethylene with a heavily branched skeletal structure. Here, the radical site abstracts hydrogen from the growing chain, and the radical site shifts from the end (a primary carbon center) to anywhere within the growing chain (a secondary carbon center). Consequently, the part of the chain from the...
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Bipolaron recombination in conjugated polymers.

Zhen Sun1, Sven Stafström

  • 1Department of Physics, Chemistry and Biology, Linköping University, SE-58183 Linköping, Sweden. zhesu@ifm.liu.se

The Journal of Chemical Physics
|August 25, 2011
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Summary
This summary is machine-generated.

Electron-electron interactions and electric fields influence bipolaron recombination in coupled polymer chains. Coulomb interactions prevent recombination in strong regimes, while weak interactions lead to localized excited states.

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Area of Science:

  • Condensed Matter Physics
  • Polymer Science
  • Theoretical Chemistry

Background:

  • Bipolarons are key charge carriers in conducting polymers.
  • Understanding bipolaron interactions is crucial for organic electronics.
  • Previous models often simplified electron-electron interactions.

Purpose of the Study:

  • To investigate the scattering and recombination of negative and positive bipolarons.
  • To analyze the role of electron-electron interactions and external electric fields.
  • To determine the recombination pathways and their yields.

Main Methods:

  • Utilized the Su-Schrieffer-Heeger model.
  • Incorporated electron-electron interactions and the Brazovskii-Kirova symmetry breaking term.
  • Applied an external electric field to a system of two coupled polymer chains.

Main Results:

  • Coulomb interactions were found to inhibit bipolaron recombination.
  • Weak Coulomb interactions resulted in recombination into a localized excited state.
  • Strong Coulomb interactions prevented bipolaron recombination entirely.
  • Identified four primary recombination channels: biexciton formation, excited polaron/free carrier pairs, and exciton/free carrier triplets.

Conclusions:

  • Electron-electron interactions play a critical role in dictating bipolaron recombination dynamics.
  • The strength of Coulomb interactions determines the possibility and outcome of bipolaron recombination.
  • The study provides a comprehensive understanding of bipolaron scattering and recombination pathways in complex polymer systems.