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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

UV–Vis Spectroscopy of Conjugated Systems

UV–Vis Spectroscopy of Conjugated Systems

Organic compounds with conjugated double bonds show strong absorption features in the UV–visible region of the electromagnetic spectrum attributed to π → π* electronic excitations. Generally, a UV–vis absorption spectrum is recorded as a plot of absorbance vs wavelength. The wavelength of maximum absorbance, which manifests as a peak in the absorption spectrum, is denoted as λmax.
One of the factors influencing λmax is the extent...

Thermal Electrocyclic Reactions: Stereochemistry

Thermal Electrocyclic Reactions: Stereochemistry

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.

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.

¹H NMR: Complex Splitting

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied...

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Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

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Study of Photoselectivity in Linear Conjugated Chromophores Using the XMS-CASPT2 Method.

Saumik Sen^1,2, Xavier Deupi^1,3,2

¹Condensed Matter Theory Group, Laboratory for Theoretical and Computational Physics, Center for Scientific Computing, Theory, and Data, Paul Scherrer Institute, 5232 Villigen, Switzerland.

ACS Physical Chemistry Au

|December 5, 2024

Summary

This study reveals that light-induced structural changes in molecules, or photoisomerization, favor specific double bond rotations in retinal-like models. Understanding this selectivity is key for biological vision and designing new photochemical switches.

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Area of Science:

Photochemistry
Computational Chemistry
Molecular Biophysics

Background:

Photoisomerization is essential for biological chromophores like retinal in opsins, which are vital for vision.
The selectivity of photoisomerization (which double bond isomerizes) depends on molecular properties and environment.

Purpose of the Study:

Investigate photoisomerization selectivity in linear conjugated chromophores using computational methods.
Analyze simple molecular models resembling retinal to understand governing factors.

Main Methods:

Employed the extended multistate complete active space second-order perturbation theory (XMS-CASPT2) method.
Analyzed electronic energies, intramolecular charge separation, and conical intersection topographies in the gas phase.

Main Results:

The photoproduct from rotation around the double bond near the Schiff base is energetically favored.
Topographic differences at conical intersections indicate distinct photodynamics for different photoproducts.
Multiphoton excitation primarily leads to reversion to the initial configuration, not rotation around other double bonds.

Conclusions:

Provides new insights into the photodynamics of photoisomerizing double bonds in π-conjugated systems.
Findings offer perspectives for understanding biological chromophores and designing photochemical switches for molecular electronics and phototherapy.