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Cycloaddition Reactions: MO Requirements for Photochemical Activation

Cycloaddition 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.

Cycloaddition Reactions: MO Requirements for Thermal Activation

Cycloaddition Reactions: MO Requirements for Thermal Activation

Thermal cycloadditions are reactions where the source of activation energy needed to initiate the reaction is provided in the form of heat. A typical example of a thermally-allowed cycloaddition is the Diels–Alder reaction, which is a [4 + 2] cycloaddition. In contrast, a [2 + 2] cycloaddition is thermally forbidden.

Cycloaddition Reactions: Overview

Cycloaddition 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.

Photochemical Electrocyclic Reactions: Stereochemistry

Photochemical 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

Infrared (IR) Spectroscopy: Overview

Infrared (IR) Spectroscopy: Overview

When electromagnetic radiation passes through a material, atoms or molecules transition from a lower to a higher energy state by absorbing radiation corresponding to the energy difference between the two states. The absorption of infrared (IR) radiation causes transitions between vibrational energy levels in a molecule. Therefore, IR spectroscopy is a useful analytical tool for determining the molecular structure of molecules.
Different compounds display unique properties due to their...

Thermal and Photochemical Electrocyclic Reactions: Overview

Thermal 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.

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Related Experiment Video

Updated: Sep 14, 2025

Ultrafast Time-resolved Near-IR Stimulated Raman Measurements of Functional π-conjugate Systems

Ultrafast Time-resolved Near-IR Stimulated Raman Measurements of Functional π-conjugate Systems

Published on: February 10, 2020

Photoinduced Intramolecular [2 + 2] Cycloaddition Investigated with Transient Infrared Spectroscopy.

Valerie S Winkler¹, Caroline G Cramer¹, Joseph A Fournier¹

¹Department of Chemistry, Washington University in St. Louis, St. Louis, Missouri 63130, United States.

The Journal of Physical Chemistry. A

|July 24, 2025

Summary

Visible light drives organic reactions, but mechanisms are unclear. Ultrafast spectroscopy reveals a photoinitiated [2 + 2] cycloaddition reaction proceeds via vibrational relaxation, internal conversion, and intersystem crossing in under 1 nanosecond.

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Area of Science:

Photochemistry
Organic Synthesis
Spectroscopy

Background:

Visible light photochemistry is a growing synthetic tool in organic and polymer chemistry.
Understanding the dynamics and mechanisms of photodriven reactions is challenging.
Photoinitiated intramolecular [2 + 2] cycloaddition reactions are important synthetic transformations.

Purpose of the Study:

To elucidate the dynamics and kinetics of a photoinitiated intramolecular [2 + 2] cycloaddition reaction.
To investigate the reaction mechanism using ultrafast transient infrared spectroscopy.
To determine the time scales of key steps in the reaction pathway.

Main Methods:

Ultrafast transient infrared spectroscopy (100 fs to 1 ns).
Visible light excitation at 430 nm.
Global fitting analysis to a four-step sequential reaction model.

Main Results:

Vibrational signatures of the bicyclo[3.2.0]heptane-2,4-dione product appeared within ~300 ps.
Fast vibrational relaxation in the S3 state (~160 fs) followed by internal conversion (~3 ps).
Formation of a key intermediate via intersystem crossing to the triplet manifold (~30 ps).
Rate-limiting product formation with a time constant of ~260 ps.

Conclusions:

The reaction proceeds through a multi-step mechanism involving electronic state relaxation and intersystem crossing.
Ultrafast spectroscopy provides critical insights into the dynamics of photochemical reactions.
The study clarifies the kinetic pathway of a visible-light-driven intramolecular [2 + 2] cycloaddition.