The Jablonski diagram gives the best explanation for the phenomenon of Fluorescence and Phosphorescence.
What is Fluorescence?
The light absorbed may be re-emitted almost instantaneously in one or more steps. This phenomenon is known as Fluorescence.
What is Phosphorescence?
The light absorbed is given out slowly and even long after the removal of the source of light. This phenomenon is called phosphorescence.
To understand this diagram, we need to define some terminology. Absorption of light photon electron absorbing molecule jump from S0 to S1, S2 or S3. For each singlet excited state (S1, S2, S3…) is a corresponding triplet excited state (T1, T2, T3…) shown in the Jablonski diagram.
The molecule, whether in a singlet or triplet excited state, is said to be activated,
A0 + hν → A*
A0 = Molecule in ground state
A* = Molecule in an excited state
The activated molecule is in the excited state, and the ground state loses its energy through the following processes.
Non-Radiative Diagram
This transition involves the return of the activated molecule from the higher excited state (S3, S2 or T3, T2) to the first excited state (S1 or T1); these transitions do not involve the emission of any radiation, and it’s called non-radiative transition.
The transition does not involve any radiations, known as non-radiative transition. The energy of an activated molecule is dissipated in the form of heat through molecular collisions; this is known as internal conversion (IC), and it occurs in less than 10-11 seconds.
The molecule may also lose energy through another process called intersystem crossing (ISC). The transition between the states of different spins, i.e. different multiplicity. For example, from S2 to T2 or S1 to T1.
Radiative Transitions
These transitions involve the return activated molecule from the singlet excited state S1 and Triplet excited state T1 to the ground state S0. Such transitions are by the emission of radiation. The transition from the S1 to S0 state is allowed and occurs in 10-8 seconds. The emissions of this transition are called Fluorescence.
The transition from triplet excited state T1 to the ground state S0 is the forbidden transition. This transition is called phosphorescence.
The phosphorescence lifetimes are much longer, being of the order of 10-3 seconds or greater.
The Jablonski diagram depicting various photophysical processes
