Consequences of Light Absorption – The Jablonski Diagram

All about the Light Absorption’s theory on the basis of Jablonski diagram.

According to the Grotthus – Draper Law of photo-chemical activation:

Only that light which is absorbed by a system, can bring a photo-chemical change.

However it is not true that all the kind of light(s) that are absorbed could bring a photo-chemical change. The absorption of light may result in a number of other phenomena as well.

  • For instance, the light absorbed may cause only a decrease in the intensity of the incident radiation. This event is governed by the Beer-Lambert Law.
  • Secondly, the light absorbed may be re-emitted almost instantaneously, within $10^{-8}$ seconds, in one or more steps. This phenomenon is well-known as fluorescence.
  • Sometimes the light absorbed is given out slowly and even long after the removal of the source of light. This phenomenon is known as phosphorescence.

The phenomena of fluorescence and phosphorescence are best explained with the help of the Jablonski Diagram.

What is Jablonski’s Diagram?
Jablonski Diagram

In order to understand Jablonski diagram, we first need to go through some basic facts. Many molecules have an even number of electrons and thus in the ground state, all the electrons are spin paired. The quantity $ \mathbf {2S+1} $ , where $ S $ is the total electronic spin, is known as the spin multiplicity of a state. When the spins are paired $ \uparrow \downarrow $ as shown in the figure, the upward orientation of the electron spin is cancelled by the downward orientation so that total electronic spin $ \mathbf {S=0} $ . That makes spin multiplicity of the state 1.

$ s_1= + \frac {1}{2}$ ; $ s_2= – \frac {1}{2}$ so that $ \mathbf{S}=s_1+s_2 =0$ .
Hence, $ \mathbf {2S+1}=1 $

Thus, the spin multiplicity of the molecule is 1. We express it by saying that the molecule is in the singlet ground state.

Spin Orientation on the absortion of a ligh photon

When by the absorption of a photon of a suitable energy $ h \nu $ , one of the paired electrons goes to a higher energy level (excited state), the spin orientation of the single electrons may be either parallel or anti-parallel. [see image]

• If spins are parallel, $ \mathbf {S=1} $ or $ \mathbf {2S+1=3} $ i.e., the spin multiplicity is 3. This is expressed by saying that the molecule is in the triplet excited state.
• If the spins are anti-parallel, then $ \mathbf{S=0} $ so that $ \mathbf {2S+1=1} $ which is the singlet excited state, as already discussed.

See, since the electron can jump to any of the higher electronic states depending upon the energy of the photon absorbed, we get a series of singlet excited states, $ {S_n} $  and a series of triplet excited state $ {T_n}$where $ n =1, 2, 3 \ldots $ . Thus $ S_1, , S_2, , S_3,  \ldots $ etc are respectively known as first singlet excited states, second singlet excited states and so on. Similarly, in $ T_1, , T_2,, ….. $, they are respectively known as first triplet excited state, second triplet excited state and so on.

Make sure, you are not confused in $ \mathbf{S}$ & $ S_n $.