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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

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: 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.

Photoreceptors and Visual Pathways

Photoreceptors and Visual Pathways

At the molecular level, visual signals trigger transformations in photopigment molecules, resulting in changes in the photoreceptor cell's membrane potential. The photon's energy level is denoted by its wavelength, with each specific wavelength of visible light associated with a distinct color. The spectral range of visible light, classified as electromagnetic radiation, spans from 380 to 720 nm. Electromagnetic radiation wavelengths exceeding 720 nm fall under the infrared category,...

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...

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Design, Synthesis, and Photochemical Properties of Clickable Caged Compounds

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Published on: October 15, 2019

Mechanism Inversion in Visible Light-Induced Photoclick Reactions.

Youxin Fu^1,2, Jingwen Zhou^1,2, Xinyi Zou¹

¹College of Science, Nanjing Forestry University, Nanjing 210037, P.R. China.

Journal of the American Chemical Society

|September 19, 2025

Summary

Researchers tuned photoclick reactions using substituted phenanthrenequinone derivatives. Substituent changes altered reaction pathways and rates, offering control over photochemical applications.

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Area of Science:

Photochemistry
Organic Chemistry
Materials Science

Background:

Photoclick chemistry enables precise control in photochemical transformations for applications like bioimaging and material functionalization.
A clear structure-property-mechanism relationship is lacking for photoclick systems, hindering rational design.

Purpose of the Study:

To develop a strategy for tuning the reactivity and mechanism of photoclick reactions involving 9,10-phenanthrenequinone (PQ) and electron-rich alkenes (ERAs).
To investigate the impact of substituents on PQ derivatives on reaction pathways and kinetics.

Main Methods:

Synthesis of 2,2'-substituted PQ derivatives with electron-withdrawing groups (EWGs) and electron-donating groups (EDGs).
Experimental studies including kinetic measurements and transient absorption spectroscopy.
Theoretical calculations to elucidate reaction mechanisms.

Main Results:

Reactivity via direct coupling decreased, while triplet-triplet energy transfer pathway increased with EWG to EDG substitution.
Excited-state energy-level inversion between 1nπ* and 1ππ* states was identified as the cause, affecting intersystem crossing.
Reaction pathways were modulated by solvent polarity.

Conclusions:

Established a robust platform for rational tuning of PQ-ERA photoclick reactions using visible light.
Provided valuable mechanistic insights into photoclick reaction control.
Offered a strategy for controlling photochemical applications in complex environments.