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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

Properties of Enantiomers and Optical Activity

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It is essential to understand the difference between chiral and achiral interactions and the implications thereof in optical activity and their applications. Just as our feet, which are chiral, interact uniquely with chiral objects, such as a pair of shoes, but identically with achiral socks, enantiomers of a molecule exhibit different properties only when they interact with other chiral media. An example of a significant implication from this facet is the phenomenon known as optical activity,...

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Chirality is the most intriguing yet essential facet of nature, governing life’s biochemical processes and precision. It can be observed from a snail shell pattern in a macroscopic world to an amino acid, the minutest building block of life. Most of the snails around the world have right-coiled shells because of the intrinsic chirality in their genes. All the amino acids present in the human body exist in an enantiomerically pure state, except for glycine - the sole achiral amino acid.

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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.

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Related Experiment Video

Updated: Jan 11, 2026

An Electrochemical Cholesteric Liquid Crystalline Device for Quick and Low-Voltage Color Modulation

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Tuning the Circularly Polarized Reflection from Cholesteric Hydroxypropyl Cellulose Using Molecular Photoswitches.

Fathy Hassan^1,2, Chun Lam Clement Chan¹, Thomas G Parton^1,3

¹Yusuf Hamied Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, UK.

Angewandte Chemie (International Ed. in English)

|November 10, 2025

Summary

This summary is machine-generated.

Researchers demonstrate light-controlled color tuning in cellulose-based liquid crystals using molecular switches. This innovation offers new possibilities for advanced photonic displays and smart materials.

Keywords:

Cholesteric liquid crystals Controlled polarization Hydroxypropyl cellulose Light‐responsive colors Molecular switches

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Area of Science:

Materials Science
Polymer Chemistry
Optics

Background:

Light-controlled molecular liquid crystals are crucial for optical devices.
Cellulose-based cholesteric polymers offer unique structural color properties.
Photo-control of cellulose-based cholesteric polymers remains underexplored.

Purpose of the Study:

To investigate the photo-responsive behavior of hydroxypropyl cellulose (HPC) cholesteric mesophases.
To explore the impact of achiral and chiral molecular switches on photonic properties.
To achieve independent control over reflected and transmitted colors in cellulose-based materials.

Main Methods:

Incorporation of achiral and chiral photoswitches into HPC cholesteric mesophases.
Monitoring changes in photonic bandgap under varying concentrations, illumination, and temperature.
Analysis of trans-cis photoisomerization and chiral interactions.

Main Results:

An achiral photoswitch enabled reversible color tuning via photoisomerization.
Chiral switches induced significant changes in pitch and polarized reflection due to complex chirality interplay.
A combination of chiral switch and spiropyran allowed independent control of reflected and transmitted colors.

Conclusions:

Molecular switches can effectively control the optical properties of cellulose-based cholesteric liquid crystals.
Chiral switches offer complex interactions influencing photonic behavior.
Independent color control opens avenues for cellulose-based photonic displays, smart labels, and anti-counterfeiting technologies.