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Related Concept Videos

Photochemical Electrocyclic Reactions: Stereochemistry01:26

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
Woodward–Hoffmann Selection Rules and Microscopic Reversibility01:34

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...
ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH301:11

ortho–para-Directing Activators: –CH3, –OH, –⁠NH2, –OCH3

All ortho–para directors, excluding halogens, are activating groups. These groups donate electrons to the ring, making the ring carbons electron-rich. Consequently, the reactivity of the aromatic ring towards electrophilic substitution increases. For instance, the nitration of anisole is about 10,000 times faster than the nitration of benzene. The electron-donating effect of the methoxy group in anisole activates the ortho and para positions on the ring and stabilizes the corresponding...
Radicals: Electronic Structure and Geometry01:07

Radicals: Electronic Structure and Geometry

This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
Accordingly, the structure of a trivalent radical lies between the geometries of carbocations and carbanions. An sp2-hybridized carbocation is trigonal planar, while an sp3-hybridized carbanion is trigonal pyramidal. Here, the difference in geometry is...
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

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 Activation01:12

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.

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Related Experiment Video

Updated: Jun 17, 2026

Light-driven Molecular Motors on Surfaces for Single Molecular Imaging
08:40

Light-driven Molecular Motors on Surfaces for Single Molecular Imaging

Published on: March 13, 2019

Bright functional rotaxanes.

Xiang Ma1, He Tian

  • 1Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai, PR China.

Chemical Society Reviews
|December 22, 2009
PubMed
Summary
This summary is machine-generated.

Researchers are creating advanced rotaxanes with exciting functional properties. This review highlights recent developments in bright rotaxanes for applications in fluorescence, logic gates, and biological systems.

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Fluorescence Lifetime Imaging of Molecular Rotors in Living Cells

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Last Updated: Jun 17, 2026

Light-driven Molecular Motors on Surfaces for Single Molecular Imaging
08:40

Light-driven Molecular Motors on Surfaces for Single Molecular Imaging

Published on: March 13, 2019

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
10:44

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals

Published on: April 19, 2019

Fluorescence Lifetime Imaging of Molecular Rotors in Living Cells
09:45

Fluorescence Lifetime Imaging of Molecular Rotors in Living Cells

Published on: February 9, 2012

Area of Science:

  • Supramolecular Chemistry
  • Materials Science

Background:

  • Rotaxanes are mechanically interlocked molecules with significant research interest.
  • Recent years have seen a surge in the development of functional rotaxanes.

Purpose of the Study:

  • To provide a tutorial review of recent advances in bright rotaxanes.
  • To highlight functional properties and novel synthetic strategies for rotaxanes.

Main Methods:

  • Focus on functional rotaxanes with specific properties.
  • Illustration of novel synthetic protocols for rotaxane construction.

Main Results:

  • Detailed overview of fluorescent rotaxanes.
  • Discussion of rotaxanes in logic gates, information storage, and gelation.
  • Exploration of rotaxanes on solid surfaces, sensory applications, and biological relevance.

Conclusions:

  • Bright rotaxanes represent a dynamic area of supramolecular chemistry.
  • Emerging applications showcase the versatility and potential of functional rotaxanes.