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Related Concept Videos

Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

1.8K
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
1.8K
Aromatic Hydrocarbon Cations: Structural Overview01:18

Aromatic Hydrocarbon Cations: Structural Overview

2.8K
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...
2.8K
Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

2.3K
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.
2.3K
Cycloaddition Reactions: MO Requirements for Photochemical Activation01:12

Cycloaddition Reactions: MO Requirements for Photochemical Activation

2.0K
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.
2.0K
Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.0K
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.
2.0K
Five-Membered Heterocyclic Aromatic Compounds: Overview01:13

Five-Membered Heterocyclic Aromatic Compounds: Overview

3.8K
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,...
3.8K

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Towards Designer Photocatalysts: Structure-Property Relationships in 2,6-Diaryl-pyryliums.

Jenna Konzal1, McKenna Murley1, Alaina Wolter1

  • 1Department of Chemistry, University of Wisconsin-Platteville, 1 University Plaza, Platteville, WI 53818, USA.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|November 17, 2024
PubMed
Summary

Researchers established quantitative structure-property relationships for organic photocatalysts. This work enables rational design of new photocatalysts by tuning their photophysical and electrochemical properties for organic synthesis.

Keywords:
Excited-statePhotocatalystPhotoredoxPyryliumStructure-property

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Area of Science:

  • Photocatalysis
  • Organic Chemistry
  • Materials Science

Background:

  • Fully organic photocatalysts offer unique reactivity and avoid transition metals.
  • Understanding redox states and structural control is vital for organic photocatalyst design.
  • Detailed structure-property relationships for organic photocatalysts are scarce in literature.

Purpose of the Study:

  • To establish linear free-energy relationships for photophysical and electrochemical properties of 2,6-diarylpyryliums.
  • To enable rational and predictive design of organic photocatalysts.
  • To provide foundational tools for organic synthesis applications.

Main Methods:

  • Investigated photophysical properties including electronic absorption and emission maxima.
  • Analyzed electrochemical properties, focusing on ground and excited state reduction potentials.
  • Employed computational modeling to identify and correct systematic errors in redox potential calculations.

Main Results:

  • Demonstrated tunable electronic absorption (83 nm) and emission (102 nm) maxima.
  • Observed intramolecular charge transfer (ICT) with polarizable heavy-atom substitution.
  • Established a strong linear dependence of ground state reduction potentials on substituent electronics.
  • Achieved control over excited state reduction potential (E*red) over a ~1000 mV range.

Conclusions:

  • Quantitative structure-property relationships were identified for 2,6-diarylpyryliums.
  • These relationships provide a foundation for rational design of organic photocatalysts.
  • The findings facilitate the development of advanced photocatalytic systems for organic synthesis.