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[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
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.
¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
Diels–Alder Reaction: Characteristics of Dienes01:29

Diels–Alder Reaction: Characteristics of Dienes

The Diels–Alder reaction brings together a diene and a dienophile to form a six-membered ring. Both components have unique characteristics that influence the rate of the reaction.
Characteristics of the diene
Conformation
The simplest example of a diene is 1,3-butadiene, an acyclic conjugated π system. At room temperature, the molecule exists as a mixture of s-cis and s-trans conformers by virtue of rotation around the carbon–carbon single bond. Although the s-trans isomer is more stable, the...
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

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 with both...

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Related Experiment Video

Updated: May 21, 2026

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization
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Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization

Published on: November 29, 2018

Aza[7]helicene Diimide: Backbone-Controlled Excited-State Charge Transfer and Electronic Coupling.

Niklas Martin1, Vincenzo Brancaccio1, Jörg-M Neudörfl1

  • 1Department of Chemistry and Biochemistry, Institute of Organic Chemistry, University of Cologne, 50939 Cologne, Germany.

Organic Letters
|May 20, 2026
PubMed
Summary
This summary is machine-generated.

Researchers synthesized a novel carbazole-embedded aza[7]helicene diimide (aza[7]HDI). This backbone-engineered molecule shows enhanced electronic communication and tunable optical properties, offering new possibilities for chiral materials.

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Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids

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Last Updated: May 21, 2026

Photogeneration of N-Heterocyclic Carbenes: Application in Photoinduced Ring-Opening Metathesis Polymerization
12:19

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Published on: November 29, 2018

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
10:44

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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
08:04

Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids

Published on: May 27, 2020

Area of Science:

  • Organic Chemistry
  • Materials Science
  • Photophysics

Background:

  • Chiral donor-acceptor helicenes are crucial for advanced optical applications.
  • Existing push-pull [7]helicene diimides (HDIs) with outer-rim substitution have limitations in electronic communication.
  • Engineering the helicene backbone offers a new avenue for property modulation.

Purpose of the Study:

  • To synthesize a backbone-engineered [7]helicene diimide (aza[7]HDI) incorporating a carbazole unit.
  • To investigate the impact of backbone donor incorporation on excited-state properties and electronic communication.
  • To explore the potential of aza[7]HDI in chiral optoelectronic materials.

Main Methods:

  • Synthesis of carbazole-embedded aza[7]helicene diimide (aza[7]HDI).
  • Spectroscopic analysis (fluorescence, solvatochromism, circularly polarized luminescence).
  • Electrochemical and quantum-chemical calculations.

Main Results:

  • The synthesized aza[7]HDI exhibits pronounced excited-state charge transfer.
  • Strong solvatochromic fluorescence and circularly polarized luminescence were observed.
  • Enhanced fluorescence quantum yields and a large excited-state dipole-moment change were achieved.
  • Electrochemical and quantum-chemical analyses confirmed strengthened through-space coupling.

Conclusions:

  • Backbone donor incorporation is an effective strategy for tuning excited-state polarization in chiral donor-acceptor helicenes.
  • Aza[7]HDI demonstrates superior electronic communication compared to outer-rim substituted analogs.
  • The findings open new avenues for designing advanced chiral optoelectronic materials.