The Jablonski Diagram
A Jablonski diagram is the simplest way to show radiative and non-radiative energy transitions between the electronic and vibrational states of a molecule.
In a Jablonski diagram, lines representing the different possible energy levels are stacked in two columns, with energy on the vertical axis. The levels are distributed horizontally according to energy state multiplicity.
Jablonski diagrams are frequently used in optical spectroscopy, particularly fluorescence spectroscopy. They are named after Polish physicist named Aleksander Jablonski, who introduced the concept in the 1930s to provide a visual representation and a theoretical framework for understanding the energy transitions and relaxation processes that occur in molecules after the absorption of light. The Jablonski diagram helped explain the mechanisms behind light-induced processes, contributing to a deeper understanding of fluorescence and phosphorescence spectroscopy.
How to Draw Energy States on a Jablonski Diagram
An example Jablonski diagram (without any energy transitions) is shown below. On the diagram, bold lines represent the base energy of each state (v0). Additional vibrational energy states (v1, v2, v3 ,etc) are represented by the thinner lines.
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 on if total spin angular momentum has been conserved. If spin has been conserved, the electron is said to be in a singlet excited state (S1). If 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.
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How to Show Energy Transitions on a Jablonski Diagram
Energy transitions are shown on Jablonski diagrams using either straight or wavy arrows. 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.
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.
The Jablonski diagram above shows six different types of transitions.
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
Vibrational relaxation 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 spectroscopy or IR spectroscopy.
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. It is represented on a Jablonski diagram 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.
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 Jablonski diagram above shows an electron moving from the excited singlet state (S1) into the excited triplet state (T1). Often intersystem crossing results in phosphorescence emission.
Phosphorescence is a type of fluorescence in which an electron relaxes into the ground state via emission of a photon. However, unlike in 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
|Fluorescence||10-9 – 10-7||UV-Vis/Optical spectroscopy|
|Intersystem Crossing||10-8 – 10-3|
|Phosphorescence||10-4 – 10-1||UV-Vis/Optical spectroscopy|
Example Energy Transitions on Jablonski 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 on the Jablonski diagram) 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).