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Related Concept Videos

¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

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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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Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

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Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
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Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

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In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must...
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Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)

1.0K
Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the...
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Spin–Spin Coupling: One-Bond Coupling01:17

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949
Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
949
NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

1.3K
The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
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Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
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Magic-Angle Exciton Coupling in a Molecular Solid: Optical Consequence of Null-Coulombic Coupling.

Tapan Ghosh1, Shant Chhetri1, Sumona Ghosh1

  • 1Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur, Nadia, West Bengal 741246, India.

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|October 11, 2024
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Summary

Magic angle configurations in molecular solids can lead to unique optical properties. This study reveals high fluorescence efficiency in CF3DPT solids due to specific electronic coupling, offering insights into excited state processes.

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Area of Science:

  • Photochemistry and Photophysics
  • Materials Science
  • Solid-State Chemistry

Background:

  • Excitonic coupling is crucial for understanding excited-state dynamics in molecular aggregates.
  • Magic angle configurations typically result in weak dipolar Coulombic coupling between chromophores.
  • Documented instances of magic angle excitonic coupling and its optical consequences are rare.

Purpose of the Study:

  • To investigate the optical properties of CF3DPT solids, particularly focusing on excitonic coupling in a magic angle configuration.
  • To demonstrate a rare experimental observation of magic angle excitonic coupling and its impact on fluorescence.
  • To elucidate the interplay between Coulombic and charge transfer (CT) coupling in aggregated molecular systems.

Main Methods:

  • Synthesis and characterization of CF3DPT solids.
  • Experimental measurement of fluorescence quantum efficiency in the aggregated solid state.
  • Computational calculations to validate experimental findings on electronic coupling.

Main Results:

  • CF3DPT solids exhibited a high fluorescence quantum efficiency of 62 ± 3% in the aggregated state.
  • Experimental and computational results confirmed null-Coulombic coupling, characteristic of a magic angle configuration.
  • Significant charge transfer (CT) coupling was identified as a key factor influencing the observed optical properties.

Conclusions:

  • The study presents a rare example of magic angle excitonic coupling in CF3DPT solids.
  • The observed high fluorescence quantum efficiency is attributed to the specific electronic coupling environment, including null-Coulombic and significant CT coupling.
  • Findings provide fundamental insights into excited-state electronic processes and optical behavior in molecular aggregates with magic angle configurations.