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Related Concept Videos

Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

3.1K
Neutral hydrocarbons like cyclopentadiene with an odd number of carbon atoms and one intervening CH2 group in the ring are not aromatic. Cyclopentadiene with 4 π electrons does not satisfy the 4n + 2 π electron rule. Additionally, the intervening CH2 group is sp3 hybridized and lacks a vacant p orbital, thereby interrupting the overlap of p orbitals in a continuous manner and preventing the delocalization of π electrons throughout the ring.
Due to the absence of continuous...
3.1K
Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

4.5K
Heterocyclic aromatic compounds are cyclic compounds that are aromatic and have one or more heteroatoms—atoms other than carbon, in the ring. Depending upon the number of atoms present in the ring, they can be either five or six-membered. Examples of five-membered heterocyclic aromatic compounds include pyrrole, furan, thiophene, and imidazole. Pyrrole consists of one nitrogen atom having one lone pair of electrons. Furan and thiophene have one oxygen and one sulfur heteroatom,...
4.5K
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

3.2K
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...
3.2K
Electrophilic Aromatic Substitution: Overview01:16

Electrophilic Aromatic Substitution: Overview

11.9K
In an electrophilic aromatic substitution reaction, an electrophile substitutes for a hydrogen of an aromatic compound.
11.9K
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

3.1K
The inscribed polygon method is consistent with Hückel’s 4n + 2 rule and helps to learn whether the given cyclic compound is aromatic or not. The compound is stable and aromatic if every bonding molecular orbital (MO) is completely filled with a pair of electrons. However, if the non-bonding or antibonding orbitals are filled with electrons, the compound is unstable and not aromatic. Consider the Frost circle diagrams for cycloalkenes containing 4 to 8 carbons.
3.1K
Criteria for Aromaticity and the Hückel 4n + 2 Rule01:20

Criteria for Aromaticity and the Hückel 4n + 2 Rule

11.6K
Like benzene, cyclobutadiene and cyclooctatetraene are cyclic compounds with alternate single and double bonds. However, their chemical behavior differs from benzene, as they are unstable and not aromatic. So, what are the structural characteristics of unsaturated compounds categorized as aromatic?  
For the first time, Eric Hückel, a German chemical physicist, derived a set of structural features for a compound to be classified as aromatic. This is now known as...
11.6K

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Updated: Oct 10, 2025

Synthesis of pH Dependent Pyrazole, Imidazole, and Isoindolone Dipyrrinone Fluorophores using a Claisen-Schmidt Condensation Approach
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Porphyrinoids, a unique platform for exploring excited-state aromaticity.

Jinseok Kim1, Juwon Oh2, Atsuhiro Osuka3

  • 1Department of Chemistry, Yonsei University, Seoul 03722, Korea. dongho@yonsei.ac.kr.

Chemical Society Reviews
|December 8, 2021
PubMed
Summary
This summary is machine-generated.

Baird aromaticity describes how molecules change aromaticity in excited states, reversing ground-state rules. Porphyrinoids experimentally confirm this excited-state aromaticity reversal, advancing functional materials.

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Area of Science:

  • Photochemistry
  • Organic Chemistry
  • Materials Science

Background:

  • Baird aromaticity describes excited-state aromaticity, reversing Hückel's rules.
  • Ground-state Hückel aromatic [4n+2]π molecules become Baird aromatic [4n]π in excited states, and vice versa.
  • Aromaticity significantly influences molecular properties and reaction mechanisms.

Purpose of the Study:

  • To review experimental verification of excited-state aromaticity reversal using porphyrinoids.
  • To highlight the conceptual development and applications of Baird aromaticity.
  • To explore the role of porphyrinoids in demonstrating reversed aromaticity.

Main Methods:

  • Review of experimental studies on porphyrinoids.
  • Analysis of structural and electronic properties of porphyrinoids related to aromaticity.
  • Investigation of porphyrinoids' application in functional organic materials.

Main Results:

  • Porphyrinoids serve as key models for observing excited-state aromaticity reversal (Baird's rule).
  • Experimental evidence confirms the concept of reversed aromaticity in excited states.
  • Porphyrinoids facilitate the development and application of Baird aromaticity in novel materials.

Conclusions:

  • Porphyrinoids are crucial for understanding and demonstrating excited-state aromaticity reversal.
  • Baird aromaticity is a significant factor in excited-state properties and processes.
  • The study of porphyrinoids advances the application of aromaticity concepts in functional organic materials.