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Related Concept Videos

Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

1.9K
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
1.9K
Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

2.2K
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.
2.2K
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

2.5K
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.
2.5K
Cycloaddition Reactions: Overview01:16

Cycloaddition Reactions: Overview

2.9K
Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.
2.9K
Cycloaddition Reactions: MO Requirements for Thermal Activation01:16

Cycloaddition Reactions: MO Requirements for Thermal Activation

3.7K
Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.
3.7K
Pericyclic Reactions: Introduction01:17

Pericyclic Reactions: Introduction

8.5K
Pericyclic reactions are organic reactions that occur via a concerted mechanism without generating any intermediates. The reactions proceed through the movement of electrons in a closed loop to form a cyclic transition state, where rearrangement of the σ and π bonds yields specific products.
Pericyclic reactions can be classified into three categories: electrocyclic reactions, cycloaddition reactions, and sigmatropic rearrangements. Electrocyclic reactions and sigmatropic...
8.5K

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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals
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Photoreactive Crystals Exhibiting [2 + 2] Photocycloaddition Reaction and Dynamic Effects.

Bibhuti Bhusan Rath1, Jagadese J Vittal1

  • 1Department of Chemistry, National University of Singapore, 117543 Singapore.

Accounts of Chemical Research
|May 2, 2022
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Summary

Solid-state photoreactions, particularly [2+2] cycloadditions, enable solvent-free synthesis with precise structural control. Dynamic crystals exhibiting photosalient effects are advancing crystal adaptronics and smart materials.

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Area of Science:

  • Solid-state chemistry
  • Crystal engineering
  • Materials science

Background:

  • Solvent-free reactions via solid-state transformations offer environmental benefits and precise structural control.
  • Single-crystal to single-crystal (SCSC) reactions, especially [2+2] cycloadditions, provide detailed structural insights through X-ray crystallography.
  • Crystal engineering principles guide the alignment of reactive groups for predictable solid-state reactivity.

Purpose of the Study:

  • To provide an overview of advancements in solid-state [2+2] cycloaddition reactions.
  • To illustrate strategies for fabricating advanced solid-state materials through photoreactivity.
  • To highlight the role of weak interactions, guest solvents, and mechanical grinding in solid-state transformations.

Main Methods:

  • Exploiting topochemical principles and crystal engineering for controlled photoreactions.
  • Utilizing single-crystal X-ray crystallography (SCXRD) for structural characterization of SCSC reactions.
  • Investigating UV-induced structural transformations and photosalient effects in dynamic crystals.

Main Results:

  • SCSC photoreactions yield quantitative, regio- and stereospecific products unattainable in solution.
  • Dynamic crystals exhibiting photosalient effects demonstrate light-to-mechanical work conversion.
  • Solid-state reactions can lead to advanced materials like optical storage, photomechanical systems, and crystalline polymers.

Conclusions:

  • Solid-state [2+2] cycloadditions are crucial for developing novel materials with tunable properties.
  • The field of crystal adaptronics leverages dynamic photoreactive crystals for smart actuating applications.
  • Rational design approaches, considering factors beyond topochemistry, are key to advancing solid-state materials.