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Molecular Spectroscopy: Absorption and Emission01:14

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Molecules possess discrete energy levels called quantum states. Unlike atoms, which have simpler energy levels, molecules possess additional rotational and vibrational energy levels.  Each energy level is separated by an energy gap, with the gaps between adjacent electronic, vibrational, and rotational levels varying significantly. The three types of energy levels in a diatomic molecule are shown in Figure 1.
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Organic compounds with conjugated double bonds show strong absorption features in the UV–visible region of the electromagnetic spectrum attributed to π → π* electronic excitations. Generally, a UV–vis absorption spectrum is recorded as a plot of absorbance vs wavelength. The wavelength of maximum absorbance, which manifests as a peak in the absorption spectrum, is denoted as λmax.
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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.
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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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In Ultraviolet–Visible (UV–Vis) spectroscopy, the absorption of electromagnetic radiation is used to probe the electronic structure of molecules. This technique provides insights into molecular electronic transitions, particularly the movement of electrons between different molecular orbitals. Radiation is absorbed if the energy of the electromagnetic radiation passing through the molecule is precisely equal to the energy difference between the excited and ground states. During this...
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Two-Photon Absorption in Conjugated Energetic Molecules.

Josiah A Bjorgaard, Andrew E Sifain1, Tammie Nelson

  • 1Department of Physics and Astronomy, University of Southern California , Los Angeles, California 90089-0484, United States.

The Journal of Physical Chemistry. A
|June 4, 2016
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Summary
This summary is machine-generated.

Researchers studied conjugated energetic molecules (CEMs) and their optical properties. Modifying molecular structure impacts explosive performance and optical absorption, enabling potential optical initiation of CEMs.

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Area of Science:

  • Computational chemistry
  • Materials science
  • Photochemistry

Background:

  • Conjugated energetic molecules (CEMs) are synthesized with tunable properties.
  • Molecular structure influences heat of formation, oxygen balance, sensitivity, and explosive strength.
  • Optical absorption properties, including one-photon absorption (OPA) and two-photon absorption (TPA), are sensitive to molecular structure.

Purpose of the Study:

  • To investigate the structure-property relationships of CEMs concerning their OPA and TPA characteristics.
  • To explore how molecular modifications affect electronic structure and optical absorption.
  • To assess the potential for optical initiation of CEMs.

Main Methods:

  • Time-dependent density functional theory (TD-DFT) calculations were employed.
  • Calculated vertical excitation energies were compared with experimental data.
  • Natural transition orbitals (NTOs) were analyzed to understand electronic transitions.

Main Results:

  • Calculated excitation energies showed good agreement with experimental values.
  • Significant peak two-photon absorption (TPA) intensities (around 10^2 GM) were predicted.
  • Molecular modifications, such as adding oxygen or functional groups, altered electronic structure, OPA, and TPA, while improving oxygen balance.

Conclusions:

  • CEMs exhibit significant nonlinear absorption properties.
  • Strategic molecular design can optimize both energetic performance and optical characteristics.
  • The findings suggest the possibility of controlled optical initiation of CEMs via photochemical pathways.