Photosensitized reactions B.Sc. SEMESTER-5

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. T he 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:- Fluorescence Phosphorescnce 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 :- Solution ofquinine sulphate on exposure to visible light, exhibits blue fluorescence. 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 :- Sulphates of calcium, barium, strontitum 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.

13. 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.

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

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

16. 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.