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Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

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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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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.
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[3,3] Sigmatropic Rearrangement of 1,5-Dienes: Cope Rearrangement01:21

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The Cope rearrangement is classified as a [3,3] sigmatropic shift in 1,5-dienes, leading to a more stable, isomeric 1,5-diene. The reaction involves a concerted movement of six electrons, four from two π bonds and two from a σ bond, via an energetically favorable chair-like transition state.
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Crystal Field Theory - Octahedral Complexes02:58

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Crystal Field Theory
To explain the observed behavior of transition metal complexes (such as colors), a model involving electrostatic interactions between the electrons from the ligands and the electrons in the unhybridized d orbitals of the central metal atom has been developed. This electrostatic model is crystal field theory (CFT). It helps to understand, interpret, and predict the colors, magnetic behavior, and some structures of coordination compounds of transition metals.
CFT focuses on...
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Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

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Cycloheptatriene is a neutral monocyclic unsaturated hydrocarbon that consists of an odd number of carbon atoms and an intervening sp3 carbon in the ring. The three double bonds in the ring correspond to 6 π electrons, which is a Huckel number, and therefore satisfies the criteria of 4n + 2 π electrons. However, the intervening sp3 carbon disrupts the continuous overlap of p orbitals. As a result, cycloheptatriene is not aromatic.
Removing one hydrogen from the intervening CH2 group...
3.6K
Crystal Field Theory - Tetrahedral and Square Planar Complexes02:46

Crystal Field Theory - Tetrahedral and Square Planar Complexes

48.0K
Tetrahedral Complexes
Crystal field theory (CFT) is applicable to molecules in geometries other than octahedral. In octahedral complexes, the lobes of the dx2−y2 and dz2 orbitals point directly at the ligands. For tetrahedral complexes, the d orbitals remain in place, but with only four ligands located between the axes. None of the orbitals points directly at the tetrahedral ligands. However, the dx2−y2 and dz2 orbitals (along the Cartesian axes) overlap with the ligands less than the dxy,...
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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals
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Crystal Packing-Trajectory Correlation in Topochemical Photoisomerization.

Bryan Po-Wen Chen1, Chao-Ping Hsu2,3, Joseph Jen-Tse Huang2

  • 1Department of Chemistry, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Da'an District, Taipei, 10617, Taiwan.

Angewandte Chemie (International Ed. in English)
|November 10, 2025
PubMed
Summary
This summary is machine-generated.

Solid-state photoisomerization is possible in close-packed crystals if sufficient steric freedom exists. This study reveals a correlation between local free volume and photoresponse, enabling rational design of photoswitchable materials.

Keywords:
PhotochromismPhotoisomerizationSingle crystalTopochemistry

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Area of Science:

  • Materials Science
  • Photochemistry
  • Crystallography

Background:

  • Solid-state photoisomerization typically requires loosely packed structures for molecular flexibility.
  • Understanding the relationship between crystal packing and photoreactivity is crucial for designing new materials.

Purpose of the Study:

  • To investigate solid-state photoisomerization in salicylhydrazone derivatives.
  • To establish a structure-property relationship linking molecular packing to photochromic response.
  • To develop a predictive model for solid-state photoreactivity.

Main Methods:

  • Synthesis and crystallization of salicylhydrazone derivatives.
  • X-ray crystallography to analyze crystal packing and π-π stacking.
  • Quantification of accessible volume along isomerization pathways.
  • Development of a crystal-structure based descriptor ('pedal space').

Main Results:

  • Several close-packed salicylhydrazone crystals exhibited photochromic behavior.
  • A strong correlation was found between local free volume and photoresponse.
  • A critical threshold for solid-state photoreactivity was identified.
  • Packing motifs (slipped vs. co-facial stacking) influence photochemical pathways.

Conclusions:

  • Topochemical photoisomerization is governed by crystal packing and local free volume.
  • The 'pedal space' descriptor effectively predicts solid-state photoreactivity.
  • Crystal engineering principles can guide the in silico design of photoswitchable materials.
  • This approach facilitates cost-effective development of responsive optoelectronic systems.