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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
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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.
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...
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Racemic Mixtures and the Resolution of Enantiomers02:30

Racemic Mixtures and the Resolution of Enantiomers

A racemic mixture, or racemate, is an equimolar mixture of enantiomers of a molecule that can be separated using their unique interaction with chiral molecules or media. Racemic mixtures are denoted by the (±)- prefix. This ‘optical rotation descriptor’ applies to the whole solution of a racemic mixture rather than a specific stereoisomer. Enantiomers typically have the same physical and chemical properties. Hence, they are not easily separable. However, enantiomers can exhibit different...
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Reaction centers are pigment-protein complexes that initiate energy conversion from photons to chemical entities. Therefore, photochemical reaction center is a more appropriate term that describes these complexes. The Nobel laureates Robert Emerson and William Arnold provided the first experimental evidence of photochemical reaction centers by demonstrating the participation of nearly 2,500 chlorophyll molecules for the release of just one molecule of oxygen. Despite thousands of photosynthetic...

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Determination of the Photoisomerization Quantum Yield of a Hydrazone Photoswitch
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Published on: February 7, 2022

A photoreactive crystalline quasiracemate.

Rebecca C Grove1, Steven H Malehorn, Meghan E Breen

  • 1Department of Chemistry, Eastern Illinois University, Charleston, Illinois 61920, USA.

Chemical Communications (Cambridge, England)
|September 14, 2010
PubMed
Summary

Sulfonamide cinnamic acids form hydrogen-bonded dimers. These structures enable asymmetric photodimerization in quasiracemic mixtures, offering new pathways for crystal engineering and materials science.

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Area of Science:

  • Supramolecular Chemistry
  • Organic Crystal Engineering
  • Photochemistry

Background:

  • Rational design of organic molecules is crucial for controlling self-assembly.
  • Hydrogen bonding plays a key role in the formation of ordered crystalline structures.
  • Photodimerization of olefins is a fundamental photochemical reaction with applications in materials science.

Purpose of the Study:

  • To investigate the self-assembly of rationally designed racemic and quasiracemic sulfonamide cinnamic acids.
  • To explore the formation of hydrogen-bonded dimers and their olefin alignment.
  • To study the asymmetric photodimerization of supramolecular heterodimers in quasiracemate phases.

Main Methods:

  • Crystallization of sulfonamide cinnamic acids.
  • X-ray diffraction analysis to determine crystal structures.
  • Spectroscopic methods to study photodimerization reactions.

Main Results:

  • Racemic and quasiracemic sulfonamide cinnamic acids successfully formed hydrogen-bonded dimers.
  • Neighboring olefins in the dimers exhibited coplanar alignment.
  • Quasiracemate phases contained near inversion-related motifs, forming supramolecular heterodimers.
  • These heterodimers underwent asymmetric photodimerization.

Conclusions:

  • Sulfonamide cinnamic acids are effective building blocks for creating ordered supramolecular structures.
  • The coplanar alignment of olefins facilitates photodimerization.
  • Asymmetric photodimerization in quasiracemic systems offers a route to chiral materials.