Related Concept Videos
01:26Photochemical Electrocyclic Reactions: Stereochemistry
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
01:26Thermal and Photochemical Electrocyclic Reactions: Overview
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.
01:12Cycloaddition Reactions: MO Requirements for Photochemical Activation
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.
01:16Cycloaddition Reactions: Overview
Cycloadditions are one of the most valuable and effective synthesis routes to form cyclic compounds. These are concerted pericyclic reactions between two unsaturated compounds resulting in a cyclic product with two new σ bonds formed at the expense of π bonds. The [4 + 2] cycloaddition, known as the Diels–Alder reaction, is the most common. The other example is a [2 + 2] cycloaddition.
![[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
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.
01:29Diels–Alder Reaction Forming Bridged Bicyclic Products: Stereochemistry
Diels–Alder reactions between cyclic dienes locked in an s-cis configuration and dienophiles yield bridged bicyclic products.
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Related Experiment Video
Updated: May 25, 2026
09:34
Synthesis of Information-bearing Peptoids and their Sequence-directed Dynamic Covalent Self-assembly
Published on: February 6, 2020
Electron transfer within self-assembling cyclic tetramers using chlorophyll-based donor-acceptor building blocks.
Victoria L Gunderson1, Amanda L Smeigh, Chul Hoon Kim
1Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, USA.
Journal of the American Chemical Society
|February 15, 2012
Summary
Researchers created chlorophyll-based molecules that self-assemble into cyclic tetramers. These structures show slower charge recombination, a key step for artificial photosynthesis.
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Area of Science:
- Supramolecular Chemistry
- Photochemistry
- Materials Science
Background:
- Artificial photosynthesis requires efficient light harvesting and charge separation.
- Chlorophyll derivatives offer a promising scaffold for developing light-harvesting systems.
- Donor-acceptor molecules are crucial for controlling charge transfer dynamics.
Purpose of the Study:
- To synthesize novel chlorophyll-based donor-acceptor triads.
- To investigate their self-assembly into cyclic tetramers.
- To study the photoinduced charge transfer properties and lifetimes in these assemblies.
Main Methods:
- Synthesis of chlorophyll (Chl) derivatives modified with pyromellitimide (PI) and naphthalene-1,8:4,5-bis(dicarboximide) (NDI) acceptors.
- Characterization of cyclic tetramer formation using small- and wide-angle X-ray scattering.
- Analysis of photoinduced charge transfer using femtosecond and nanosecond transient absorption spectroscopy.
Main Results:
- Successful synthesis of Chl-PI-NDI and Chl-PI-NDI(2) building blocks.
- Formation of cyclic tetramers in solution via Chl metal-ligand coordination.
- Photoexcitation of Chl leads to sequential electron transfer: Chl -> PI -> NDI.
- Charge recombination lifetimes were significantly longer in tetramers (30 ns) compared to monomers (10 ns).
Conclusions:
- Self-assembly into cyclic tetramers enhances charge separation lifetimes.
- These Chl-based supramolecular systems show potential for artificial photosynthesis.
- Structural changes induced by self-assembly are key to improving charge separation efficiency.