Name: jablonski diagram: light loss pathways
Uploaded: 2024-12-05T18:32:04.000Z
Duration: 1 min 25 s
Channel: JoVE

When a molecule absorbs photons, its electrons are excited to higher electronic and vibrational energy levels.
The excited electrons exist in a singlet state relative to the ground-state electron or in a triplet state.
Transition to the ground state involves no spin flips and occurs through a combination of photon-emitting and non-photon-emitting, radiationless processes.
Deactivation pathways are visualized in a Jablonski diagram where S0 is the ground singlet electronic state.
The three excited electronic states are S1 and S2, the first and second electronic singlets, and T1, the first electronic triplet state.
In radiationless deactivation via vibrational relaxation, the molecule loses energy while moving to a lower vibrational level within the same electronic state.
It causes a shift in the fluorescence band toward lower frequencies or longer wavelengths than the absorption band, causing Stokes shift.
In internal conversion, a molecule moves from an excited electronic state to a lower energy state with the same spin.
External conversion transfers excess energy to the surrounding solvent or sample matrix.
Conversely, intersystem crossing involves transitions between singlet and triplet states.

deactivation processes: jablonski diagram

Deactivation Processes: Jablonski Diagram

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 of most molecules in a solution at room temperature. The upper lines represent vibrational energy levels of three excited electronic states: the first and second electronic singlet states, and the first electronic triplet state.
Each of these four electronic states is associated with numerous vibrational energy levels. Absorption transitions can occur from the ground singlet electronic state to various vibrational levels of the excited singlet electronic states. The triplet transition involves a change in multiplicity, so it has a very low probability of occurrence. An excited molecule can return to its ground state through several mechanistic steps. Two of these steps, fluorescence, and phosphorescence, involve the emission of a photon, while others are radiationless processes. The favored route to the ground state is the one that minimizes the lifetime of the excited state. Therefore, if deactivation by fluorescence is rapid compared to the radiationless processes, such emission is observed. Conversely, fluorescence is either absent or less intense if a radiationless path has a more favorable rate constant.
Molecules excited to the first and second electronic singlet states rapidly lose any excess vibrational energy and relax to the ground vibrational level of that electronic state, through a nonradiational process called vibrational relaxation. The term internal conversion describes intermolecular processes that leave the molecule in a lower-energy electronic state without the emission of radiation. These processes are neither well-defined nor well-understood but are often highly efficient. Internal conversion can occur between two states of the same multiplicity (singlet-singlet or triplet-triplet), especially when two electronic energy levels are sufficiently close for there to be an overlap in vibrational energy levels.
Intersystem crossing, another deactivation process, is a crossover between electronic states of different multiplicity. Like internal conversion, the probability of intersystem crossing is enhanced if the vibrational levels of the two states overlap. Intersystem crossing is most common in molecules that contain heavy atoms, such as iodine or bromine, due to increased spin and orbital interactions.
Vibrational relaxation, internal conversion, external conversion, and intersystem crossing are all forms of radiationless deactivation where a molecule in an excited state loses energy without emitting a photon. On the other hand, fluorescence and phosphorescence involve the emission of a photon. In both cases, the molecule returns to a lower energy electronic state, but the difference lies in the multiplicity of the states involved and the lifetime of the excited state. In summary, the deactivation processes in luminescence are complex and involve a combination of radiative and non-radiative transitions. The efficiency of these processes can greatly affect the observed luminescent properties of a material.

Luminescence, the emission of light by a substance that has absorbed energy, is a process that involves the interaction of molecules with light. The ...

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jablonski diagram: light loss pathways

Jablonski diagrams show how excited molecules lose energy after absorbing light. They are used to track both light-emitting and non-light-emitting steps in luminescence. The diagram gives a simple picture of the energy states a molecule can move through.

The lowest line in a Jablonski diagram is the ground-state energy of the molecule. This state is usually a singlet state, and it represents the condition of most molecules in solution at room temperature. The lines above it show vibrational energy levels in three excited electronic states: the first singlet state, the second singlet state, and the first triplet state.

Absorption can move a molecule from the ground singlet state to different vibrational levels in the excited singlet states. A triplet transition is much less likely because it requires a change in multiplicity. After excitation, the molecule can return toward the ground state through several deactivation steps.

Two of those steps are fluorescence and phosphorescence. Both involve photon emission, so they are radiative processes. Other pathways do not give off light and instead are radiationless processes. The route that is favored is the one that shortens the excited-state lifetime the most.

If fluorescence happens faster than the radiationless paths, the emission is observed. If a radiationless path has the better rate constant, fluorescence may be weak or absent. Molecules in the first or second singlet state also lose extra vibrational energy quickly through vibrational relaxation, which is a nonradiative process.

Internal conversion is another radiationless process. It moves a molecule to a lower-energy electronic state without emitting radiation. The process is not well defined or fully understood, but it is often very efficient. Internal conversion can occur between states with the same multiplicity, such as singlet to singlet or triplet to triplet, especially when their vibrational levels overlap.

Intersystem crossing is a crossover between states with different multiplicity. Like internal conversion, it becomes more likely when vibrational levels overlap. It is especially common in molecules with heavy atoms such as iodine or bromine because spin and orbital interactions are stronger. Vibrational relaxation, internal conversion, external conversion, and intersystem crossing are all forms of radiationless deactivation, while fluorescence and phosphorescence release light from lower-energy electronic states.

Jablonski Diagram: Light Loss Pathways

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key terms and concepts

Learn how Jablonski diagrams map fluorescence, phosphorescence, internal conversion, and intersystem crossing as molecules lose energy after absorbing light.

Jablonski Diagram: Light Loss Pathways | JoVE K12

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Jablonski Diagram: Light Loss Pathways

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