Jablonski Diagrams

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Jablonski diagrams are the simplest way to the transitions between electronic and vibrational states. The representative energy levels are arranged with energy on the vertical axis and vary horizontally according to energy state multiplicity. An example Jablonski diagram is shown below.

Bold lines represent the base energy of each state (v₀), with additional vibrational energy states in lighter grey lines above this (v₁, v₂, v₃ ,etc). The ground state is represented as S₀ as spin angular momentum S=0 in the ground state. When an electron enters an excited state, it can have different spin multiplicity, depending if total spin angular momentum has been conserved. If spin has been conserved, this is a singlet excited state (S₁). However, angular momentum is not conserved, then the electron enters a triplet energy state (T₁). The excited singlet and triplet states are represented in Jablonski diagrams with different columns, as shown in the diagram above. 

Energy Transitions on Jablonski Diagrams

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Radiative transitions (such as absorption and fluorescence) are shown with straight arrows, while non-radiative transitions (such as internal conversion and intersystem crossing) tend to be depicted as wavy arrows.

In the above Jablonski diagram, 6 different types of transitions are depicted.

They are:

1. Absorbance

This represents the absorbance of a photon by an electron. The energy of the photon is high enough that the electron can be excited into a higher energy state. Absorption between energy levels can be explored with optical spectroscopy if this transition energy is between 1.1 - 3.8 eV.

2. Vibrational relaxation

This is a non-radiative loss of energy between vibrational energy levels. This excess vibrational energy is lost as kinetic energy to other vibrational modes, either of the same molecule or of a different molecule. This energy is loss happens very rapidly (10^-14 - 10^-12 seconds)  and is often measured using Raman or IR spectroscopy.

3. Fluorescence

Fluorescence is a type of photoluminescence in which the spin state of the electron relaxes back into the ground state, and a photon is emitted. Therefore it is represented by a straight line. The spin state of the electron stays the same from the excited state to the ground state, so this is a singlet-singlet transition (S₁ → S₀). This is an allowed transition so fluorescence often occurs a very short time after the electron is excited. Fluorescence can be measured with optical spectroscopy if this transition energy is between 1.1 - 3.8 eV.

4. Internal conversion

Internal conversion is a type of non-radiative emission, where an electron moves from a higher energy excited state, to a lower energy excited state. This occurs when the vibrational modes of different electronic levels overlap. No photon is emitted and the electrons spin state remains the same throughout the transition.

5. Intersystem crossing

Intersystem crossing is another form of non-radiative emission. Unlike internal conversion, the spin state of the excited electron changes. The example show here shows an electron moving from the excited singlet state (S₁) into the excited triplet state (T₁). Often intersystem crossing results in phosphorescence emission.

6. Phosphorescence

This is a type of fluorescence in which a electron relaxes into the ground state via emission of a photon. However, unlike for fluorescence, the electron must change spin states for this to occur. This is a forbidden transition, so happens over a much longer time scale. Phosphorescence can be measured with optical spectroscopy if the transition is between 1.1 - 3.8 eV.

Time Scales for Different Energy Transitions

Transition	Time Taken (s)	How to measure?
Absorbance	10-14 - 10-12	UV-Vis/Optical spectroscopy
Vibrational relaxation	10-14 - 10-11	Time resolved spectroscopy IR spectroscopy Raman spectroscopy
Fluorescence	10-9 - 10-7	UV-Vis/Optical spectroscopy
Internal Conversion	10-15
Intersystem Crossing	10-8 - 10-3
Phosphorescence	10-4 - 10-1	UV-Vis/Optical spectroscopy

 

Example of Energy Transitions on Jablonski Diagram

This is an example of how you can represent a series of energy transition on an inert diagram. In the Jablonski diagram above, a photon is absorbed, exciting a ground state electron to an excited vibrational state of the second singlet excited state, S₂ (dark blue straight arrow pointing upwards). It then relaxes through internal conversion to an excited vibrational state of S₁ (faint blue wavy arrow) and further relaxes to the ground vibrational state of S₁ through vibrational relaxation (LHS light blue wavy arrow). The electron then undergoes intersystem crossing (green wavy arrow) to an excited vibrational state of the first excited triplet state (T₁) and undergoes vibrational relaxation once more (RHS light blue wavy arrow) to the T₁ vibrational ground state. After a time (can be seconds or even hours), the electron relaxes back down to an excited vibrational state of the singlet ground state, S₀, via phosphorescence (angled blue arrow).

You can see here that Jablonski diagrams provide a convenient way to represent energy transitions in a molecule or system. However, it is important to note that most of the time, these diagrams are purely schematic and do not represent these energy levels quantitatively.

  

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