PHOTOSENSITIZED REACTIONS / PHOTOPHYSICAL PROCESS / FLUOROSCENCE / PHOSPHORESCENCE / CHEMILUMINESCENCE

1. Photosensitized reactions
CECH-509
UNIT – 4 B
BY – Ms MAYURI R SOMPURA
HJD INSTITUTE

2. INTRODUCTION :-
• In many photochemical reactions, the reactant molecule doesnot
absorb the radiation required for the reaction.
• Hence, the reaction is not possible.
• In such cases, reactions may still occur if a forigen species such as
mercury vapour is present.
• The mercury atom absorbs the incident radiation and transfers that
energy to reactant molecule which is activated.
• Thus, reaction occurs.

3. INTRODUCTION :-
Photosensitizer - A species which can absorb and transfer radiant
energy for the activation of the reactant molecule.
This reaction so caused is called as photosensitized reaction.

4. The role of mercury vapours:-
The mercury atom absorbs the incident radiation and gets excited.
The excited atom collides with a reactant molecule and transfers the
excitation energy.
Mercury returns back to its unactivated state.

5. Example :-
Reaction between hydrogen and oxygen
Reaction between hydrogen and carbon monoxide

6. Photophysical process:-
If the absorbed radiation is not used to cause a chemical change, its re-
emitted as light of longer wavelength.
The three such processes are:-
1. Fluorescence
2. Phosphorescnce
3. Chemiluminescence

7. Fluorescence :-
Certain molecules when exposed to light radiation of short
wavelength (high frequency) they emit light of longer wavelength.
This process is known as fluorescence.
 substance that exhibit fluorescence is known as florescent substance.
Fluorescence stops as soon as the incident radiation is cut off.

8. Fluorescence :-
Example :-
1. Solution ofquinine sulphate on exposure to visible light, exhibits
blue fluorescence.
2. Solution of chlorophyll in either shows red fluorescence.

9. Fluorescence :-
Explanation :-
When a molecule absorbs high energy radiation, it is excited to higher
energy states.
Then it emits excess energy through several transitions to the ground
state.
Thus the excited molecule emits light of longer frequency.
The colour depends on the wavelength of light emitted.

10. Phosphorescence :-
When a molecule absorbs radiation of high frequency and emits light
even after the incident radiation is cut off, the process is called
phosphorescence.
The substance which show phosphorescence is known as
phosphorescent substance.
It is chiefly caused by ultraviolet and visible light.
Generally shown by solids.

11. Phosphorescence :-
Examples :-
1. Sulphates of calcium, barium, strontitum
2. Fluorescein in boric acid shows phosphorescence in a blue region at
5700 A wavelength.

12. Phosphorescence :-
Explanation :-
A molecule absorbs light radiation and gets excited.
While returning to ground state, it emits energy of lower wavelength.
In doing so, the excited molecule passes from one series of excited
states to other and gets trapped.
This shows emission of light which persists even after removal of the
light source.
Thus phosphorescence could be designated as delayed fluorescence.

14. Chemiluminescence :-
Some chemical reactions are accompanied by the emission of vivible
light at ordinary temperature.
The emission of light as the result of chemical action is called as
chemiluminescence.
The reaction is referred as chemiluminescent reaction.
Such a reaction is the reverse of phototchemical reaction which
proceeds by absorption of light.
The light emitted bin this reaction is known as ‘cold light’ because it
is produced at an ordinary temperature.

15. Chemiluminescence :-
Examples :-
1. The glow of fireflies due to the aerial oxidation of luciferon (a
protein) in the presence of enzyme luciferase.

16. Chemiluminescence :-
Examples :-
2. The oxidation of 5-aminophthalic cyclic hydrazide (luminol) by
hydrogen peroxide in alkaline solution producing bright green light.

17. Chemiluminescence :-
Explanation :-
In chemilumiscent reaction, the energy released in the reaction makes
the product molecuoe electronically excited.
The excited molecule then gives up its excess energy as visible light
while reverting to ground state.