Show by means of an energy diagram the reason that the...

Name: Problem 9, Chapter 12: Advanced Organic Chemistry. Part A. Structure and Mechanisms
Uploaded: 2021-11-13T19:54:11-08:00
Duration: 1 min 43 s
Channel: Mir Afzal
Description: Show by means of an energy diagram the reason that the energy of light emitted from an excited electronic state by fluorescence or phosphorescence is of lower energy than the exciting radiation. Would you expect the shift in energy to be more pronounced for fluorescence or phosphorescence? Explain?

Show by means of an energy diagram the reason that the energy of light emitted from an excited electronic state by fluorescence or phosphorescence is of lower energy than the exciting radiation. Would you expect the shift in energy to be more pronounced for fluorescence or phosphorescence? Explain?

Key Concepts

Energy Diagrams

An energy diagram visually represents the electronic energy levels of a molecule and the transitions between them. It typically shows the ground state, excited states, and the processes by which energy is absorbed or released, including nonradiative relaxation and radiative emissions. This diagram helps conceptualize why the emitted light has lower energy than the absorbed light, as energy is lost through internal processes before photon emission.

Electronic Excitation and Relaxation

When a molecule absorbs light, an electron is promoted from a lower, ground state to a higher, excited state. However, before the electron returns to the ground state, it often undergoes nonradiative relaxation processes, such as vibrational relaxation, where energy is dissipated as heat. This loss of energy during relaxation ensures that when the electron emits a photon, the energy of the light is lower than that of the absorbed photon.

Fluorescence

Fluorescence is the process where an excited electron returns to the ground state through a radiative transition, emitting a photon in the process. The emission typically occurs from the lowest vibrational level of the excited state, after rapid vibrational relaxation. Because some energy has been lost nonradiatively before light emission, the fluorescence photon is red-shifted compared to the absorbed photon.

Phosphorescence

Phosphorescence involves a forbidden transition where the excited electron undergoes intersystem crossing to a triplet state before returning to the ground state. This process is usually slower than fluorescence and involves even more energy loss due to the longer time before emission. As a result, the emitted light in phosphorescence tends to be of even lower energy compared to fluorescence, due to the additional energy dissipation incurred during the spin-forbidden transition.

Stokes Shift

The Stokes shift refers to the phenomenon where the emitted light has a longer wavelength (and hence lower energy) than the absorbed light. This shift occurs primarily due to nonradiative relaxation processes, such as vibrational relaxation, that occur prior to photon emission. It is a key concept in understanding the energy loss between absorption and emission in both fluorescence and phosphorescence.

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