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Related Concept Videos

Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical 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
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Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal 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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Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

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Some cycloaddition reactions are activated by heat, while others are initiated by light. For example, a [2 + 2] cycloaddition between two ethylene molecules occurs only in the presence of light. It is photochemically allowed but thermally forbidden.
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Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal 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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The light reactions of photosynthesis assume a linear flow of electrons from water to NADP+. During this process, light energy drives the splitting of water molecules to produce oxygen. However, oxidation of water molecules is a thermodynamically unfavorable reaction and requires a strong oxidizing agent. This is accomplished by the first product of light reactions: oxidized P680 (or P680+), the most powerful oxidizing agent known in biology. The oxidized P680 that acquires an electron from the...
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Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals01:17

Electron Paramagnetic Resonance (EPR) Spectroscopy: Organic Radicals

2.4K
Ideally, an unpaired electron shows a single peak in the EPR spectrum due to the transition between the two spin energy states. However, coupling interactions can occur between the spins of the unpaired electron and any neighboring spin-active nuclei. This hyperfine coupling results in hyperfine splitting, where the EPR signal is split into multiplets. The signals split into 2nI + 1 peaks, where n is the number of equivalent nuclei and I is the nuclear spin. These splitting patterns provide...
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Related Experiment Video

Updated: Jun 11, 2025

[DPEPhosbcpCu]PF6: A General and Broadly Applicable Copper-Based Photoredox Catalyst
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Visualizing back electron transfer in eosin Y photoredox catalysis.

Kai Gu1, Wenqiao Zhou1, Chunming Liu1,2

  • 1School of Polymer Science and Polymer Engineering, University of Akron, Akron, OH, 44325, USA. chunmingliu@uakron.edu.

Chemical Communications (Cambridge, England)
|October 9, 2024
PubMed
Summary

Back electron transfer in eosin Y photoredox catalysis was visualized using single-molecule fluorescence. This photoblinking mechanism, observed with alkyl bromides but not tertiary amines, clarifies polymerization catalysis.

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Area of Science:

  • Photochemistry
  • Catalysis
  • Polymer Science

Background:

  • Eosin Y (EY) is a widely used photosensitizer in photoredox catalysis.
  • Understanding electron transfer processes is crucial for optimizing catalytic efficiency.
  • Back electron transfer (BET) is a key process influencing catalytic cycles.

Purpose of the Study:

  • To visualize and understand the mechanism of back electron transfer (BET) in eosin Y (EY) photoredox catalysis.
  • To investigate the role of different quenchers in the BET process.
  • To elucidate the implications of BET for EY-catalyzed photoinduced atom transfer radical polymerizations.

Main Methods:

  • Single-molecule fluorescence microscopy was employed to visualize EY fluorescence.
  • Photoinduced electron transfer (PET) and subsequent BET were monitored by observing photoblinking of single EY molecules.
  • The influence of common quenchers, including alkyl bromides and a tertiary amine, on BET was systematically studied.

Main Results:

  • Back electron transfer (BET) was directly visualized through the photoblinking of single eosin Y (EY) molecules.
  • BET was observed to occur in the presence of alkyl bromide quenchers.
  • Notably, BET was not detected when using a tertiary amine as the quencher.

Conclusions:

  • The study provides direct visualization of BET in EY photoredox catalysis via single-molecule fluorescence.
  • The findings elucidate the mechanism of EY-catalyzed photoinduced atom transfer radical polymerizations by highlighting the role of BET.
  • The developed single-molecule fluorescence method offers a versatile platform for studying BET in other photo-emissive catalytic systems.