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Thermal and Photochemical Electrocyclic Reactions: Overview

Thermal and Photochemical Electrocyclic Reactions: Overview

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.

Cycloaddition Reactions: Overview

Cycloaddition Reactions: Overview

Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.

Photochemical Electrocyclic Reactions: Stereochemistry

Photochemical Electrocyclic Reactions: Stereochemistry

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

Cycloaddition Reactions: MO Requirements for Photochemical Activation

Cycloaddition Reactions: MO Requirements for Photochemical Activation

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.

Thermal Electrocyclic Reactions: Stereochemistry

Thermal Electrocyclic Reactions: Stereochemistry

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.

Cycloaddition Reactions: MO Requirements for Thermal Activation

Cycloaddition Reactions: MO Requirements for Thermal Activation

Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.

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Updated: Nov 8, 2025

Synthesis of Antiviral Tetrahydrocarbazole Derivatives by Photochemical and Acid-catalyzed C-H Functionalization via Intermediate Peroxides CHIPS

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Published on: June 20, 2014

Optical Cycling Functionalization of Arenes.

Claire E Dickerson, Han Guo, Guo-Zhu Zhu

The Journal of Physical Chemistry Letters

|April 20, 2021

Summary

Researchers explored complex molecules for quantum science applications. They found that large aromatic molecules, like coronene, can be used for optical cycling, enabling new quantum sensing and information processing capabilities.

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Patterning via Optical Saturable Transitions - Fabrication and Characterization

Patterning via Optical Saturable Transitions - Fabrication and Characterization

Published on: December 11, 2014

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Synthesis of Antiviral Tetrahydrocarbazole Derivatives by Photochemical and Acid-catalyzed C-H Functionalization via Intermediate Peroxides CHIPS

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Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

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Published on: July 17, 2020

Patterning via Optical Saturable Transitions - Fabrication and Characterization

Patterning via Optical Saturable Transitions - Fabrication and Characterization

Published on: December 11, 2014

Area of Science:

Quantum Information Science
Quantum Sensing
Molecular Quantum Technology

Background:

Closed, laser-induced optical transitions enable state preparation and measurement in quantum applications.
Complex molecules offer potential for enhanced quantum science capabilities, but size limits are unknown.

Purpose of the Study:

To investigate the feasibility of using increasingly complex molecular constructs for optical cycling.
To determine the size and structural limits of ligands in molecular optical cycling.

Main Methods:

Exploration of Calcium-ligand (Ca-O-L) molecular constructs, with arenes as ligands (L).
Analysis of Franck-Condon factor (FCF) diagonality with varying ligand size and substituents.
Computational investigation of electronic state interactions between ligands and the cycling center.

Main Results:

Optical cycling is maintained for arene ligands as large as coronene (CaOC24H11).
Electron-withdrawing groups on ligands can enhance the FCF.
Ligands larger than approximately 7 rings disrupt FCF diagonality by altering electronic state ordering.

Conclusions:

Optical cycling is achievable with complex arene ligands, expanding possibilities for molecular quantum technologies.
A design principle for molecular optical cycling is proposed based on ligand size and electronic properties.