Related Concept Videos
01:26Photochemical Electrocyclic Reactions: Stereochemistry
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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
Selection Rules: Photochemical Activation
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01:26Thermal and Photochemical Electrocyclic Reactions: Overview
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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.
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01:12Cycloaddition Reactions: MO Requirements for Photochemical Activation
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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.
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01:17Thermal Electrocyclic Reactions: Stereochemistry
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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.
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.
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02:13Energy Diagrams, Transition States, and Intermediates
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Free-energy diagrams, or reaction coordinate diagrams, are graphs showing the energy changes that occur during a chemical reaction. The reaction coordinate represented on the horizontal axis shows how far the reaction has progressed structurally. Positions along the x-axis close to the reactants have structures resembling the reactants, while positions close to the products resemble the products. Peaks on the energy diagram represent stable structures with measurable lifetimes, while...
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01:16UV–Vis Spectroscopy: Molecular Electronic Transitions
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In Ultraviolet–Visible (UV–Vis) spectroscopy, the absorption of electromagnetic radiation is used to probe the electronic structure of molecules. This technique provides insights into molecular electronic transitions, particularly the movement of electrons between different molecular orbitals. Radiation is absorbed if the energy of the electromagnetic radiation passing through the molecule is precisely equal to the energy difference between the excited and ground states. During this...
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Structural Engineering of Two-Dimensional Covalent Organic Frameworks for Visible-Light-Driven Organic
Haoran Liu1,2, Chunzhi Li2,3, He Li2
1Key Laboratory of Theoretical Organic Chemistry and Functional Molecule (Ministry of Education), School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China.
ACS Applied Materials & Interfaces
|April 11, 2020
Summary
Covalent organic frameworks (COFs) with specific substituents and triazine skeletons show enhanced photocatalytic activity. These novel organic semiconductors are recyclable and offer potential for designing efficient materials for organic transformations.
Area of Science:
- Materials Science
- Organic Chemistry
- Photocatalysis
Background:
- Covalent organic frameworks (COFs) are emerging as visible light-responsive organic semiconductors.
- Understanding key parameters influencing COF photocatalytic properties is crucial for their application.
Purpose of the Study:
- To investigate factors affecting photocatalytic properties of [3+3] COFs.
- To explore the impact of substituents and framework composition on COF performance.
Main Methods:
- Synthesis of a series of [3+3] COFs with varying compositions.
- Evaluation of COFs in visible-light-driven reductive dehalogenation and aerobic cross-dehydrogenative coupling reactions.
Main Results:
- Hydroxyl (-OH) substituents narrowed the band gap compared to -H and -CF3.
- Triazine skeletons enhanced photocatalytic activity by improving charge separation.
- A COF with both -OH and triazine exhibited the highest activity due to narrow band gap, efficient charge separation, and high conductivity.
- COFs demonstrated excellent recyclability without loss of crystallinity.
Conclusions:
- Substituent choice and framework design significantly impact COF photocatalytic efficiency.
- COFs offer a promising platform for developing efficient and recyclable photocatalysts for organic synthesis.