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Related Concept Videos

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...
Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
Thermal Sigmatropic Reactions: Overview01:16

Thermal Sigmatropic Reactions: Overview

Sigmatropic rearrangements are a class of pericyclic reactions in which a σ bond migrates from one part of a π system to another. These are intramolecular rearrangements where the total number of σ and π bonds remain unchanged.
Sigmatropic shifts are classified based on an order term [i, j ], where i and j indicate the number of atoms across which each end of the σ bond migrates. Below are examples of a [3,3] sigmatropic shift in 1,5-hexadiene, referred to as...
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
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.
Thermal Electrocyclic Reactions: Stereochemistry01:17

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.

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Updated: Jun 3, 2026

Assembling Molecular Shuttles Powered by Reversibly Attached Kinesins
08:04

Assembling Molecular Shuttles Powered by Reversibly Attached Kinesins

Published on: January 26, 2019

Solvatochromic rotaxane molecular shuttles.

Duygu Deniz Günbaş1, Leszek Zalewski, Albert M Brouwer

  • 1Van't Hoff Institute for Molecular Sciences, University of Amsterdam, P.O. Box 94157, 1090 GD Amsterdam, The Netherlands.

Chemical Communications (Cambridge, England)
|March 25, 2011
PubMed
Summary

This study introduces a fluorescent bistable rotaxane. Its macrocyclic ring

Area of Science:

  • Supramolecular Chemistry
  • Materials Science

Background:

  • Rotaxanes are molecular machines with potential applications in nanoscale devices.
  • Bistable rotaxanes offer controllable states for advanced functionalities.

Purpose of the Study:

  • To describe a novel strongly fluorescent bistable rotaxane.
  • To demonstrate spectral response based on the rotaxane's molecular structure.

Main Methods:

  • Synthesis of a bistable rotaxane incorporating a solvatochromic fluorophore.
  • Spectroscopic analysis to correlate ring position with fluorescence emission.

Main Results:

  • The rotaxane exhibits strong fluorescence.
  • The position of the macrocyclic ring significantly influences the solvatochromic fluorophore's spectral properties.

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  • A clear spectral response was observed based on the bistable states.

    • Conclusions:

    • The developed rotaxane is a promising candidate for molecular switches and sensors.
    • The spectral response provides a mechanism for reading the state of the molecular machine.