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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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03:02Colors and Magnetism
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Color in Coordination Complexes
When atoms or molecules absorb light at the proper frequency, their electrons are excited to higher-energy orbitals. For many main group atoms and molecules, the absorbed photons are in the ultraviolet range of the electromagnetic spectrum, which cannot be detected by the human eye. For coordination compounds, the energy difference between the d orbitals often allows photons in the visible range to be absorbed and emitted, which is seen as colors by the human...
When atoms or molecules absorb light at the proper frequency, their electrons are excited to higher-energy orbitals. For many main group atoms and molecules, the absorbed photons are in the ultraviolet range of the electromagnetic spectrum, which cannot be detected by the human eye. For coordination compounds, the energy difference between the d orbitals often allows photons in the visible range to be absorbed and emitted, which is seen as colors by the human...
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02:52Stereoisomerism
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Isomerism in Complexes
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric isomers, in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH3)4Cl2]+ ion has two isomers (Figure 1) In the cis...
Isomers are different chemical species that have the same chemical formula.
Transition metal complexes often exist as geometric isomers, in which the same atoms are connected through the same types of bonds but with differences in their orientation in space. Coordination complexes with two different ligands in the cis and trans positions from a ligand of interest form isomers. For example, the octahedral [Co(NH3)4Cl2]+ ion has two isomers (Figure 1) In the cis...
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![[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction](/_next/image?url=https%3A%2F%2Fcloudfront.jove.com%2Ffiles%2Fmedia%2Fscience-education%2Fscience-education-thumbs%2F12417.jpg&w=3840&q=75)
01:16[4+2] Cycloaddition of Conjugated Dienes: Diels–Alder Reaction
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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.
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Cyclodextrin-Confined Supramolecular Lanthanide Photoswitch.
Hua-Jiang Yu1, Haoran Wang1, Fang-Fang Shen1
1College of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin, 300071, P. R. China.
Small (Weinheim an Der Bergstrasse, Germany)
|May 19, 2022
Summary
This study introduces a novel azobenzene-based nanosystem that achieves switchable lanthanide luminescence. By utilizing host-guest complexation, the system overcomes azobenzene
Keywords:
azobenzeneconformational confinementcyclodextrinlanthanide complexesswitchable luminescenceMore Related Videos
Area of Science:
- Supramolecular Chemistry
- Materials Science
- Photochemistry
Background:
- Azobenzene photoisomerization controls morphology and biological processes.
- Azobenzene's light-quenching ability hinders photoluminescent nanoconstruct design.
Purpose of the Study:
- To develop an azobenzene-derived nanosystem with switchable photoluminescence.
- To explore its function as a supramolecular lanthanide photoswitch.
Main Methods:
- Utilizing metal chelation between lanthanide ions (Eu3+, Tb3+) and 2,6-pyridinedicarboxylic acid as the light-emitting center.
- Employing α-cyclodextrin to confine trans-azobenzene, enabling luminescence recovery.
- Reversibly switching luminescence off by expelling cis-azobenzene from α-cyclodextrin via light irradiation.
Main Results:
- Azobenzene's disordered motion initially suppresses lanthanide fluorescence.
- Hydrophobic cavity of α-cyclodextrin immobilizes trans-azobenzene, restoring lanthanide luminescence.
- Luminescence is reversibly switched off upon cis-azobenzene expulsion.
Conclusions:
- Host-guest complexation and metal-ligand coordination enable photoswitchable luminescence in azobenzenes.
- This approach holds promise for creating advanced light-responsive smart materials.