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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

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.

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

Electrocyclic reactions, cycloadditions, and sigmatropic rearrangements are concerted pericyclic reactions that proceed via a cyclic transition state. These reactions are stereospecific and regioselective. The stereochemistry of the products depends on the symmetry characteristics of the interacting orbitals and the reaction conditions. Accordingly, pericyclic reactions are classified as either symmetry-allowed or symmetry-forbidden. Woodward and Hoffmann presented the selection criteria for...

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: Long-Range Coupling

¹H NMR: Long-Range Coupling

The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene...

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Related Experiment Video

Updated: Mar 14, 2026

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells

Published on: March 19, 2017

Abnormal Chiral Coupling for Efficient and Stable Reduced-Dimensional Perovskite Emitters.

Zicheng Li¹, Xinyu Duan¹, Hanting Meng^2,3

¹State Key Laboratory of Extreme Photonics and Instrumentation, College of Optical Science and Engineering, Zhejiang University, Zhejiang, Hangzhou 310027, China.

|March 13, 2026

Summary

Chiral coupling in reduced-dimensional halide perovskite emitters boosts luminescence efficiency and stability. This breakthrough enables highly efficient and stable perovskite optoexcitonic devices without external passivators.

Keywords:

chiral perovskites excited-state transfer exciton−LO phonon coupling low-threshold CW lasing structural order

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Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation

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Published on: October 1, 2019

Area of Science:

Materials Science
Optoelectronics
Solid-State Chemistry

Background:

Reduced-dimensional halide perovskites (RDPs) are crucial for optoexcitonic devices.
Achieving high luminescence efficiency, stability, and phase purity simultaneously remains a challenge.
Existing methods often rely on external passivators, limiting intrinsic material improvements.

Purpose of the Study:

To introduce a novel design principle for RDP emitters.
To enhance luminescence efficiency, stability, and phase purity without external passivators.
To investigate the role of chiral coupling in RDPs.

Main Methods:

Incorporation of chiral benzene halide RDPs.
Construction of nanoscale lattices and long-range superstructures.
Analysis of defect reduction, exciton-phonon scattering, and excited-state transfer.

Main Results:

An abnormal chiral coupling effect was observed.
Phase purity improved, defects reduced, and exciton-phonon scattering weakened.
Photoluminescence quantum yield increased by over 35% compared to achiral counterparts.
Stable, low-threshold continuous-wave lasing was achieved for over 0.5 hours at room temperature.

Conclusions:

Chiral coupling is an effective strategy for enhancing RDP performance.
This approach offers a new design principle for defect control, phase purity, and microstructure.
Enables development of highly efficient and stable perovskite optoexcitonic devices.