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Related Concept Videos

Halogenation of Alkenes02:46

Halogenation of Alkenes

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Halogenation is the addition of chlorine or bromine across the double bond in an alkene to yield a vicinal dihalide. The reaction occurs in the presence of inert and non-nucleophilic solvents, such as methylene chloride, chloroform, or carbon tetrachloride.
Consider the bromination of cyclopentene. Molecular bromine is polarized in the proximity of the π electrons of cyclopentene. An electrophilic bromine atom adds across the double bond, forming a cyclic bromonium ion intermediate.
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Photochemical Electrocyclic Reactions: Stereochemistry01:26

Photochemical Electrocyclic Reactions: Stereochemistry

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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
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Thermal and Photochemical Electrocyclic Reactions: Overview01:26

Thermal and Photochemical Electrocyclic Reactions: Overview

2.9K
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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Thermal Electrocyclic Reactions: Stereochemistry01:17

Thermal Electrocyclic Reactions: Stereochemistry

2.5K
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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Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene01:14

Electrophilic 1,2- and 1,4-Addition of X2 to 1,3-Butadiene

3.4K
Electrophilic addition of halogens to alkenes proceeds via a cyclic halonium ion to form a 1,2-dihalide or a vicinal dihalide.
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Electrophilic Addition to Alkynes: Halogenation02:38

Electrophilic Addition to Alkynes: Halogenation

10.0K
Introduction
Halogenation is another class of electrophilic addition reactions where a halogen molecule gets added across a π bond. In alkynes, the presence of two π bonds allows for the addition of two equivalents of halogens (bromine or chlorine). The addition of the first halogen molecule forms a trans-dihaloalkene as the major product and the cis isomer as the minor product. Subsequent addition of the second equivalent yields the tetrahalide.
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Updated: Jan 14, 2026

Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films
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Low Pressure Vapor-assisted Solution Process for Tunable Band Gap Pinhole-free Methylammonium Lead Halide Perovskite Films

Published on: September 8, 2017

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Light-Driven Intraoctahedral Halide Isomerization in Two-Dimensional Mixed Halide Perovskites.

Wenxin Mao1,2, Enamul Haque3, Stephanie A Bird4

  • 1Department of Chemical Engineering and Biotechnology, University of Cambridge, Philippa Fawcett Drive, Cambridge CB3 0AS, U.K.

Journal of the American Chemical Society
|January 13, 2026
PubMed
Summary
This summary is machine-generated.

Two-dimensional metal halide perovskites exhibit light-driven, reversible halide-ion isomerization within single octahedra. This intraoctahedral switching tunes optoelectronic properties, enabling new possibilities for reconfigurable devices.

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Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation
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Monovalent Cation Doping of CH3NH3PbI3 for Efficient Perovskite Solar Cells
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Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation
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Area of Science:

  • Materials Science
  • Solid-State Chemistry
  • Quantum Optics

Background:

  • Two-dimensional (2D) metal halide perovskites are promising for quantum light emission and neuromorphic computing.
  • Their functionality is often linked to quantum confinement and tunable optoelectronic properties.
  • Unique opportunities exist for engineering functionality at the subunit-cell level by exploiting distinct halide sites within octahedra.

Purpose of the Study:

  • To investigate light-driven, reversible halide-ion isomerization in single-crystalline 2D perovskites.
  • To understand the mechanism of intraoctahedral halide switching and its impact on optoelectronic properties.
  • To explore the potential of this phenomenon for advanced optoelectronic applications.

Main Methods:

  • Hyperspectral imaging
  • In situ X-ray diffraction
  • First-principles calculations (Density Functional Theory)

Main Results:

  • Demonstrated reversible halide-ion isomerization within individual PbX6(4-) octahedra in BA2PbBrxI4-x (x=1-3).
  • Observed modulation of the optical bandgap by ~0.1 eV due to intraoctahedral halide site switching.
  • Estimated a reversible electronic bandgap shift of up to ~0.5 eV, attributed to valence band character redistribution.

Conclusions:

  • Uncovered a mechanism of structurally encoded, site-selective photoisomerization in 2D perovskites.
  • Showcased intraoctahedral halide switching as a novel strategy for reconfigurable optoelectronic devices.
  • Highlighted potential applications in nonvolatile optical memory and quantum photonics.