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Related Concept Videos

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...
π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds01:14

π Electron Effects on Chemical Shift: Aromatic and Antiaromatic Compounds

In aromatic compounds, such as benzene, the circulation of (4n + 2) π-electrons sets up a diamagnetic or diatropic ring current around the perimeter of the molecule. This current induces a magnetic field that opposes the external field inside the ring and reinforces it on the outside. The protons in benzene are deshielded and exhibit high chemical shifts in the range 6.5–8.5 ppm. The shielding effect at the center of the ring is evident in complex aromatic molecules, such as annulenes. In...
Aromatic Hydrocarbon Anions: Structural Overview01:18

Aromatic Hydrocarbon Anions: Structural Overview

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 overlap of p...
Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

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, respectively.
Basicity of Heterocyclic Aromatic Amines01:25

Basicity of Heterocyclic Aromatic Amines

Heterocyclic amines, where the N atom is a part of an alicyclic system, are similar in basicity to alkylamines. Interestingly, the heterocyclic amine having a nitrogen atom as part of an aromatic ring has much less basicity than its corresponding alicyclic counterpart. For this reason, as presented in Figure 1, piperidine (pKb = 2.8) is significantly more basic than pyridine (pKb = 8.8).
Frost Circles for Different Conjugated Systems01:18

Frost Circles for Different Conjugated Systems

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.

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Synthesis of pH Dependent Pyrazole, Imidazole, and Isoindolone Dipyrrinone Fluorophores using a Claisen-Schmidt Condensation Approach
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Synthesis of pH Dependent Pyrazole, Imidazole, and Isoindolone Dipyrrinone Fluorophores using a Claisen-Schmidt Condensation Approach

Published on: June 10, 2021

Pyrrole Annulation Controls Protonation-Induced Topology Switching in π-Extended Core-Modified Figure-of-Eight

Sumit Sahoo1, Sergio Moles Quintero2, Mercedes Alonso2

  • 1School of Chemical Sciences, Indian Association for the Cultivation of Science, 2A/2B Raja S.C Mullick Road, Jadavpur, Kolkata, West Bengal 700 032, India.

Organic Letters
|June 11, 2026
PubMed
Summary

Two novel figure-of-eight hexaphyrins were synthesized to study how annulation affects protonation. Annulation preserves macrocycle topology and localizes electronic response, impacting near-infrared absorption properties.

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Last Updated: Jun 12, 2026

Synthesis of pH Dependent Pyrazole, Imidazole, and Isoindolone Dipyrrinone Fluorophores using a Claisen-Schmidt Condensation Approach
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Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units
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Preparation of a Corannulene-functionalized Hexahelicene by Copper(I)-catalyzed Alkyne-azide Cycloaddition of Nonplanar Polyaromatic Units

Published on: September 18, 2016

Area of Science:

  • Organic Chemistry
  • Supramolecular Chemistry
  • Photophysics

Background:

  • Figure-of-eight [38]hexaphyrins are macrocycles with unique topologies and tunable electronic properties.
  • Protonation can significantly alter the structure and aromaticity of macrocyclic systems.
  • Near-infrared (NIR) absorption is crucial for applications in imaging and photodynamic therapy.

Purpose of the Study:

  • To synthesize and investigate two core-modified figure-of-eight [38]hexaphyrins with varying degrees of pyrrole annulation.
  • To understand how pyrrole annulation influences protonation-induced changes in macrocycle topology and aromaticity.
  • To correlate structural and electronic modifications with near-infrared (NIR) absorption characteristics.

Main Methods:

  • Synthesis of two π-extended core-modified figure-of-eight [38]hexaphyrins.
  • Protonation studies to induce topological switching and assess aromaticity modulation.
  • Theoretical calculations including Time-Dependent Density-Functional Theory (TD-DFT).
  • Analysis using aromaticity descriptors and ring-current analysis.

Main Results:

  • The nonannulated hexaphyrin exhibited protonation-driven planarization and enhanced global aromaticity.
  • The β-annulated hexaphyrin showed a rigidified macrocycle, preserving the figure-of-eight topology upon protonation.
  • Annulation was found to localize the electronic response, with NIR Q-type bands primarily arising from local excitations.

Conclusions:

  • Pyrrole annulation effectively controls protonation-induced topology switching and aromaticity modulation in figure-of-eight hexaphyrins.
  • The observed NIR absorption is mainly attributed to local electronic excitations, modulated by annulation, rather than global aromaticity.
  • These findings provide insights into the design of macrocycles with tailored photophysical properties for specific applications.