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Related Concept Videos

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.
Intramolecular Claisen Condensation of Dicarboxylic Esters: Dieckmann Cyclization01:13

Intramolecular Claisen Condensation of Dicarboxylic Esters: Dieckmann Cyclization

Dieckmann cyclization is an intramolecular Claisen condensation of diesters. The reaction occurs in the presence of a base and generates a cyclic β-ketoester as the final product. Commonly, 1, 6 and 1, 7-diesters are preferred substrates for the reaction since the generated five, and six-membered cyclic β-keto esters are particularly more stable.
Stability of Conjugated Dienes01:28

Stability of Conjugated Dienes

Introduction
A comparison of the enthalpies of hydrogenation of dienes reveals that conjugated dienes release less heat on hydrogenation, rendering them more stable than their nonconjugated analogs.
[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction01:16

[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction

The Diels–Alder reaction is an example of a thermal pericyclic reaction between a conjugated diene and an alkene or alkyne, commonly referred to as a dienophile. The reaction involves a concerted movement of six π electrons, four from the diene and two from the dienophile, forming an unsaturated six-membered ring. As a result, these reactions are classified as [4+2] cycloadditions.
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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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals
10:35

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Published on: May 29, 2018

Electron-induced dynamics of heptathioether β-cyclodextrin molecules.

Avijit Kumar1, René Heimbuch, Kim S Wimbush

  • 1Physics of Interfaces and Nanomaterials, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.

Small (Weinheim an Der Bergstrasse, Germany)
|November 22, 2011
PubMed
Summary

Scanning tunneling microscopy reveals dynamic behavior in beta-cyclodextrin self-assembled monolayers. Electron-induced conformational changes and molecular vibrations were observed, crucial for understanding transport in these systems.

Keywords:
charge transportcyclodextrinsmolecular dynamicsscanning tunneling microscopyself-assembled monolayers

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Area of Science:

  • Surface Science
  • Nanotechnology
  • Molecular Dynamics

Background:

  • Self-assembled monolayers (SAMs) are crucial in nanotechnology.
  • Understanding molecular dynamics in SAMs is key for electronic applications.
  • Beta-cyclodextrin (β-CD) offers unique structural properties for molecular studies.

Purpose of the Study:

  • To investigate the dynamical behavior of heptathioether β-cyclodextrin SAMs on gold.
  • To explore electron-induced conformational changes and molecular vibrations within β-CD molecules.
  • To elucidate the fundamental mechanisms governing charge transport in molecular monolayers.

Main Methods:

  • Variable-temperature scanning tunneling microscopy (STM) and spectroscopy (STS).
  • Recording tunneling current-time traces with the STM feedback loop disabled.
  • Analyzing dynamics as a function of tunneling current and sample bias.

Main Results:

  • Observed rich, temperature-independent dynamical behavior in β-CD molecules.
  • Demonstrated electron-induced conformational changes driven by inelastic electron tunneling.
  • Identified well-defined current jumps attributed to molecular vibrations, even at low biases (10 mV).

Conclusions:

  • The study highlights the importance of electron-molecule interactions in SAMs.
  • Findings provide insights into vibrational excitations and conformational dynamics.
  • Results are critical for advancing the understanding of charge transport in molecular systems.