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Related Concept Videos

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

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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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Crystal Field Theory - Tetrahedral and Square Planar Complexes02:46

Crystal Field Theory - Tetrahedral and Square Planar Complexes

50.2K
Tetrahedral Complexes
Crystal field theory (CFT) is applicable to molecules in geometries other than octahedral. In octahedral complexes, the lobes of the dx2−y2 and dz2 orbitals point directly at the ligands. For tetrahedral complexes, the d orbitals remain in place, but with only four ligands located between the axes. None of the orbitals points directly at the tetrahedral ligands. However, the dx2−y2 and dz2 orbitals (along the Cartesian axes) overlap with the ligands less than...
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Woodward–Hoffmann Selection Rules and Microscopic Reversibility01:34

Woodward–Hoffmann Selection Rules and Microscopic Reversibility

4.3K
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...
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Deactivation Processes: Jablonski Diagram01:25

Deactivation Processes: Jablonski Diagram

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Luminescence, the emission of light by a substance that has absorbed energy, is a process that involves the interaction of molecules with light. The energy-level diagram, or Jablonski diagram, is a graphical representation of these interactions, illustrating the various states and transitions a molecule can undergo. In a typical Jablonski diagram, the lowest horizontal line represents the ground-state energy of the molecule, which is usually a singlet state. This state represents the energies...
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Ionic Crystal Structures02:42

Ionic Crystal Structures

21.8K
Ionic crystals consist of two or more different kinds of ions that usually have different sizes. The packing of these ions into a crystal structure is more complex than the packing of metal atoms that are the same size.
Most monatomic ions behave as charged spheres, and their attraction for ions of opposite charge is the same in every direction. Consequently, stable structures for ionic compounds result (1) when ions of one charge are surrounded by as many ions as possible of the opposite...
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Facile Synthesis of Colloidal Lead Halide Perovskite Nanoplatelets via Ligand-Assisted Reprecipitation
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Structural Reorganization Drives Exciton Relaxation Pathways in Layered 2D Ruddlesden-Popper (RP) Perovskite BA2PbI4.

Boddeda Sai Kumar1, Bikram Pal2, Potla Yedukondalu2

  • 1Department of Materials Engineering, Indian Institute of Science, Bengaluru, India.

Small (Weinheim an Der Bergstrasse, Germany)
|April 20, 2026
PubMed
Summary

Thermal annealing enhances lattice dynamics and exciton-phonon coupling in 2D halide perovskites. This study links synthesis conditions to structural changes, improving optoelectronic properties for future material design.

Keywords:
exciton‐phonon couplinghybrid organic‐inorganic 2D perovskitespolaronself‐trapped excitons (STEs)

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Area of Science:

  • Materials Science
  • Condensed Matter Physics
  • Optoelectronics

Background:

  • 2D halide perovskites exhibit strong excitonic effects and dynamic lattices, making them promising for optoelectronics.
  • Synthesis conditions, especially thermal annealing, critically influence their structural and excitonic properties.

Purpose of the Study:

  • To systematically characterize the impact of structural reorganization induced by thermal annealing in 2D Ruddlesden-Popper (RP) n-butyl ammonium lead iodide (BA2PbI4).
  • To establish a direct link between thermal processing, structural dynamics, and exciton-phonon coupling.

Main Methods:

  • Temperature-dependent X-ray diffraction (XRD)
  • Temperature-dependent photoluminescence (TDPL)
  • Temperature-dependent resonance Raman spectroscopy
  • Terahertz time-domain spectroscopy (THz-TDS)
  • Transient absorption spectroscopy (TAS)
  • First-principles DFT calculations

Main Results:

  • Annealed samples showed enhanced lattice anharmonicity, evidenced by broadened Raman modes and increased phonon absorption near 2 THz.
  • A significant increase in the oscillator strength of the 2 THz phonon mode (nearly threefold) was observed after annealing, indicating enhanced exciton-phonon coupling.
  • TDPL revealed more pronounced self-trapped exciton (STE) emission in annealed films, and TAS showed longer carrier lifetimes (1.7 ns vs. 1.1 ns), consistent with increased exciton localization.

Conclusions:

  • Thermal annealing significantly boosts lattice dynamics and exciton-phonon coupling in 2D halide perovskites.
  • This study provides a strategy for designing low-dimensional materials by tuning synthesis conditions to control structural dynamics and optoelectronic properties.