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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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The reaction of weakly electrophilic aryldiazonium (also called arenediazonium) salts with highly activated aromatic compounds leads to the formation of products with an —N=N— link, called an azo linkage. This reaction, presented in Figure 1, is known as diazo coupling and occurs without the loss of the nitrogen atoms of the aryldiazonium salt. Highly activated aromatic compounds such as phenols or arylamines favor the diazo coupling reaction. The coupling generally occurs at the para...
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01:27Vicinal Diols via Reductive Coupling of Aldehydes or Ketones: Pinacol Coupling Overview
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Wilhelm Rudolph Fittig discovered the pinacol coupling reaction in 1859. It is a radical dimerization reaction and involves the reductive coupling of aldehydes or ketones in the presence of hydrocarbon solvent to yield vicinal diols.
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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.
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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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03:45Formation of Complex Ions
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A type of Lewis acid-base chemistry involves the formation of a complex ion (or a coordination complex) comprising a central atom, typically a transition metal cation, surrounded by ions or molecules called ligands. These ligands can be neutral molecules like H2O or NH3, or ions such as CN− or OH−. Often, the ligands act as Lewis bases, donating a pair of electrons to the central atom. These types of Lewis acid-base reactions are examples of a broad subdiscipline called coordination...
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Electron Transfer Kinetics between an Electron-Accepting Ionic Liquid and Coumarin Dyes.
Marissa Saladin1, Mark Maroncelli1
1Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States.
The Journal of Physical Chemistry. B
|December 4, 2020
Summary
Electron transfer rates in ionic liquids are similar to conventional solvents, but exhibit complex dynamics. Solvent motions in ionic liquids may control electron transfer rates, especially for slower reactions.
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Area of Science:
- Physical Chemistry
- Electrochemistry
- Materials Science
Background:
- Ionic liquids (ILs) are promising electrolytes for energy applications.
- Understanding electron transfer (ET) in ILs is crucial for optimizing their performance.
- Solvent dynamics significantly influence ET rates.
Purpose of the Study:
- Investigate electron transfer dynamics between excited dyes and a pyridinium ionic liquid.
- Compare ET rates and mechanisms in ionic liquids versus conventional solvents.
- Elucidate the role of solvent dynamics in controlling electron transfer in ILs.
Main Methods:
- Time-resolved fluorescence quenching measurements.
- Utilized 7-aminocoumarin dyes and 1-butylpyridinium bis(trifluoromethylsulfonyl)imide ([Py4][Tf2N]).
- Varied the driving force for electron transfer over a 0.7 V range.
Main Results:
- Electron transfer rates ranged from 10^10 to 10^12 s^-1, increasing with driving force.
- Observed non-exponential fluorescence decays with subpicosecond to nanosecond components.
- ET rates in [Py4][Tf2N] were comparable to those in aromatic amine solvents.
- Broadly distributed emission dynamics in ILs mirrored their characteristic solvation response.
Conclusions:
- Solvent motions in ionic liquids likely play a significant role in controlling electron transfer rates.
- The observed dynamics suggest that solvent dynamics can influence energy gap or electronic coupling.
- Findings inform the use of ionic liquids as electrolytes in energy storage devices.