1. Illustration of Jablonski Diagram
Dr. N. K. Shukla
Associate Professor
Mahatma Gandhi P.G. College
Gorakhpur

2. Jablonski Diagram
It is pictorial representation of different energy states which absorb by molecule. To understand Jablonski diagram we know spin multiplicity (S= 2s + 1) of state.
Fig 1- spin Orientation on absorption of light photon

3. When the molecule absorbs energy & gets exited in higher energy level
When the molecule absorbs energy & gets exited in higher energy level. There are two possibilities.
Fig. (b) s = ½ + ½ = 1  S = 2 × = 3.
Molecule in Triplet exited state. (T).
fig. (c) s = ½ - ½ = 0  S = 2 × = 1.
Molecule in Singlet exited state. (S)
Thus, depending upon energy we get series of singlet exited state S1 ,S2 , S3 …. And triplet exited state T1, T2 , T3 ….

4. Jablonski Diagram

5. Absorbance
The first transition in the Jablonski diagrams is the absorbance of a photon of a particular energy by the molecule of interest.
Absorbance is the method by which an electron is excited from a lower energy level to a higher energy level.

6. The excited species can return to the ground state by losing all of its excess energy by any one of the following absorption
Non-Radiative Process
Radiative Process

7. Non- radiative transition:-
It involves transition from , S2 → S1 or S3 → S1 Or T2 → T1 or T3 → T1 . It does not involve emission of any radiation hence called Non- radiative transition.
It only involves emission of heat.
1. Internal Conversion (IC):-
In this process energy loss in the form of heat. It involves transition from, S3 → S2 or S2 → S1 Or T3 → T2 or T2 → T1 .
It occurs in less than second.
2. Intersystem Crossing (ISC):-
It involves transition from S3→ T3 , S2→ T2 Or S1→ T1 Both these transitions are forbidden.

8. Radiative transition:-
It involves transition from S1→ S0 or T1→ T0 . It involves emission of transition.
Two process comes under this radiative transition
Fluorescence
Phosphorescence

9. Fluorescence
The emission of transition from S1→ S0 is called Fluorescence.
It is allowed transition and occurs in 10-8 second.
The electronic transition of electron is depends on wavelength of radiation. In case of atom, wave length of Fluorescence is same as absorbed radiation is called resonance Fluorescence. While in case of molecule it longer than absorbed radiation.

10. Quenching of Fluorescence
If the excited molecules are deactivated and the fluorescence stops, the phenomenon is called ‘Quenching’.
When the activated molecules undergo a change from a singlet excited state to triplet excited state. This is called ‘internal quenching’.
When the activated molecules collide with the other molecules/quenchers which are the externally added species and transfer their energy to those molecules. This is called ‘external quenching’.

11. Phosphorescence
The emission of transition from T1→ S0 is called Phosphorescence.
It is forbidden transition.
In phosphorescence even after incident light radiation is cut off there is emission of light for some time . It is also called slow Fluorescence.
The substance which shows Phosphorescence is called fluorescent substance.
The name Phosphorescence is derived from phosphorous which glows in dark.

12. REFERENCES R. Drago: Physical method in Inorganic Chemistry, DUSAP
K.V. Reddy: Symmetry & spectroscopy of Molecules
K.K. Rastogi & Mukharjee, Fundamentals of photochemistry, Wiley eastern
G. M. Barrow, Physical Chemistry, McGraw Hill, 5th Edn., 2007.
C. N. Banwell and E. M. McCash, Fundamentals of Molecular Spectroscopy,,Tata McGraw Hill, 1995