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01:26Thermal and Photochemical Electrocyclic Reactions: Overview
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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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00:57Cationic Chain-Growth Polymerization: Mechanism
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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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01:26Photochemical Electrocyclic Reactions: Stereochemistry
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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
Selection Rules: Photochemical Activation
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01:17Thermal Electrocyclic Reactions: Stereochemistry
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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.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
Selection Rules: Thermal Activation
Conjugated systems containing an even number of π-electron pairs undergo a conrotatory ring closure. For example, thermal electrocyclization of (2E,4E)-2,4-hexadiene, a conjugated diene containing two π-electron pairs, gives trans-3,4-dimethylcyclobutene.
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01:17Ion Exchange
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Ion exchange chromatography separates charged molecules from a solution by reversibly exchanging them with mobile, or 'active', ions associated with the oppositely charged stationary phase. This method can be used to separate ions, soften and deionize water, and purify solutions. The polymers comprising the ion-exchange column are high-molecular-weight and chemically stable polymers, crosslinked to be porous and essentially insoluble. They are also functionalized with either acidic or...
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01:04Anionic Chain-Growth Polymerization: Mechanism
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The mechanism for anionic chain-growth polymerization involves initiation, propagation, and termination steps. In the initiation step, a nucleophilic anion, such as butyl lithium, initiates the polymerization process by attacking the π bond of the vinylic monomer. As a result, a carbanion, stabilized by the electron‐withdrawing group, is generated. The resulting carbanion acts as a Michael donor in the propagation step and attacks the second vinylic monomer, which acts as a Michael...
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Monitoring the Effects of Illumination on the Structure of Conjugated Polymer Gels Using Neutron Scattering
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A Reversible Structural Phase Transition by Electrochemically-Driven Ion Injection into a Conjugated Polymer.
Connor G Bischak1, Lucas Q Flagg1, Kangrong Yan2
1Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States.
Journal of the American Chemical Society
|April 2, 2020
Summary
Conjugated polymers exhibit reversible structural phase transitions during electrochemical cycling. This unique behavior in poly[2,5-bis(thiophenyl)-1,4-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benzene] (PB2T-TEG) enables non-Fickian ion transport, crucial for energy storage.
Area of Science:
- Materials Science
- Polymer Chemistry
- Electrochemistry
Background:
- Conjugated polymers are promising for electrochemical energy storage.
- Understanding their structural dynamics during ion transport is critical for performance.
- Previous studies show Fickian diffusion in similar materials.
Purpose of the Study:
- To investigate structural phase transitions in a glycolated conjugated polymer (PB2T-TEG) during electrochemical cycling.
- To elucidate the impact of these transitions on ion-polaron pair transport.
- To explore potential applications in energy storage and neuromorphic computing.
Main Methods:
- Grazing incidence wide-angle X-ray scattering (GIWAXS) to analyze crystalline phases.
- Electrochemical oxidation and ion injection in an aqueous electrolyte.
- Moving front experiments using optical microscopy and super-resolution photoinduced force microscopy (PiFM).
Main Results:
- PB2T-TEG undergoes reversible structural phase transitions upon electrochemical oxidation/reduction.
- These transitions lead to non-Fickian ion-polaron pair transport with sharp profiles.
- Transport behavior contrasts with Fickian diffusion observed in other polymers like P3MEEMT.
Conclusions:
- The observed structural phase transitions in PB2T-TEG are analogous to those in inorganic materials like LiFePO4.
- Engineering similar phase transition properties in conjugated polymers can enhance electrochemical energy storage and neuromorphic memory applications.