Jablonski diagram is used in molecular spectroscopy to illustrate the excited states of polyatomic molecules and different types of transition that take place between different states.
Jablonski diagram is a powerful tool for explaining the consequences of photochemical excitation i.e. possible transitions that take place when a molecule is photoexcited.
A typical Jablonski diagram is shown below in the figure.
The terms, key components, events, and transitions that occur after photoexcitation and make up the Jablonski diagram are explained below.
Energy levels:
More than one electronic state is likely to exist for a given molecule. Each electronic state has a group of vibrational levels. In the ground state, most of the organic molecules have all-electron pairs. When a molecule absorbs a photon, it is promoted to a higher energy state called the excited state.
Electronic states may be singlet or triplet based upon the electron spin angular momentum.
Singlet State:
Singlet states have a total angular momentum of zero. All electrons are paired in it. Molecules are diamagnetic. Singlet states don’t split in a magnetic field. Absorption of light without spin inversion produces a singlet excited state.
S0 = singlet ground state of the molecule
S1= First singlet excited state
S2= Second singlet excited state
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Sn= nth singlet excited state
Triplet State:
Triplet states have a total angular momentum of one. All electrons are unpaired in it. Molecules are paramagnetic. Triplet states split in a magnetic field. Singlet excited states may undergo spin inversion to produce triplet excited states.
T1 = First triplet excited state
T2= Second triplet excited state
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Tn= nth triplet excited state
Here, n= 1, 2, 3, 4,……
Radiative & Non-Radiative (Radiation less) Transitions:
The transitions that take place between two molecular states with absorption or emission of the photon are called radiative Transitions.
The transitions that take place between two molecular states without absorption or emission of a photon are called non-radiative transitions.
Spin-allowed and Spin- forbidden transitions:
Transitions between states of same spin multiplicity i.e. singlet to singlet or triplet to triplet are spin- allowed but transitions between states of different spin multiplicity i.e. singlet to triplet or triplet to singlet are spin- forbidden transitions.
Absorption:
The radiative transitions from a lower to a higher electronic state of a molecule take place by the adsorption of a photon. The energy of a photon is converted into the internal energy of a molecule. It is the fastest transition that takes place on the timescale order of 10-15s.
Adsorption of a photon promotes a molecule from S0 to one of the vibrational levels of the singlet excited state (S1, S2..).
Vibrational Relaxation:
The radiationless transition of higher vibrational level to lower vibrational level within the same electronic state is called vibrational relaxation. It occurs on a rapid time scale of 10-12 to 10-10 s.
Internal Conversion (IC):
Radiationless transitions between higher energy states and lower energy statess of the same spin multiplicity are called internal conversions. S3 → S2, S2 → S1, T2 → T1, etc. are internal conversions that occur rapidly (10-11 to 10-9 s).
Fluorescence:
The radiative transition between two electronic states of the same spin multiplicity is fluorescence. The emission of light during the transition of singlet excited state S1 to the ground state is fluorescence.
Intersystem Crossing:
Transitions between the singlet state to lower triplet state or vice versa is called intersystem crossing. This is the non-radiative process that takes place between isoelectronic vibrational levels belonging to electronic states of different spin multiplicity. Example: S1 to T1
The intersystem crossing is slower than the internal conversion and has importance in photochemistry. It occurs on a timescale of 10-10 to 10-6 s.
Phosphorescence:
Phosphorescence is the radiative transition from triplet state to ground state. The emission of light during the transition of T1 to S0 is phosphorescence.
The different transition processes can be related in the form of the table below.
| Transition process | The transition between states (Example) | Type of transition | The time scale in Second |
| Adsorption | S0 to S1, | Radiative | 10-15 s |
| Vibrational relaxation | V2 to V1 | Radiationless | 10-12 to 10-10 s |
| Internal Conversion | S2 to S1 | Radiationless | 10-11 to 10-9 s |
| Intersystem Crossing | S1 to T1 | Radiationless | 10-10 to 10-6 s |
| Fluorescence | S1 to S0 | Radiative | 10-10 to 10-7 s |
| Phosphorescence | T1 to S0 | Radiative | 10-7 to 10 s |