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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.

Crystal Field Theory - Octahedral Complexes

Crystal Field Theory - Octahedral Complexes

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...

Crystal Field Theory - Tetrahedral and Square Planar Complexes

Crystal Field Theory - Tetrahedral and Square Planar Complexes

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,...

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.

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Related Experiment Video

Updated: Jul 12, 2025

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

Published on: May 29, 2018

Predicting photoactivity in dithienylethene crystalline solids.

Kristin M Hutchins¹

¹Department of Chemistry, University of Missouri, 601 S. College Ave, Columbia, Missouri 65211, USA.

|October 20, 2023

Summary

This study explores light-responsive dithienylethene materials. Advances in predicting their photoactivity combine theoretical calculations with crystal structure experiments.

Keywords:

crystal engineering crystal landscape diarylethene photoswitch

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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

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Area of Science:

Materials Science
Photochemistry
Supramolecular Chemistry

Background:

Stimuli-responsive materials change properties upon external stimuli.
Dithienylethene compounds are key photochromic molecules.
Controlling photoactivity is crucial for material applications.

Purpose of the Study:

To discuss the design principles of light-responsive dithienylethene materials.
To highlight recent advancements in predicting photoactivity.
To showcase the synergy between theoretical and experimental approaches.

Main Methods:

Computational modeling and theoretical calculations.
Crystal structure landscape experiments.
Analysis of structure-property relationships.

Main Results:

Successful prediction of photoactivity through combined methods.
Identification of key structural factors influencing light response.
Demonstration of the utility of crystal structure landscape exploration.

Conclusions:

Theoretical and experimental approaches are powerful for designing photoresponsive materials.
Predictive models enhance the rational design of dithienylethene derivatives.
Further research can accelerate the development of advanced photochromic systems.