Factor that affecting Fluorescence and Quenching and MCQ asked in EXAM

Factor that affecting Fluorescence and Quenching and MCQ asked in EXAM

Factor that affecting fluorescence :-

1. Nature of the molecule :-

Molecules that have tendency to absorbed  Electromagnetic radiation (uv,IR) can only exhibit fluroscence.

Unsaturated molecules with pi(π) bond and good resonance that molecules exhibit fluroscence.

Saturated molecules with sigma(σ) bond  do not exhibit fluroscence.

2. Affect of functional group :-

Electron denoting group increase fluroscence.

Example is hydroxy,amine etc

Electron accepting group decrease fluroscence.

Example is nitro group

3. Molecular weight :-

High molecular weight atom have low intensity fluroscence.

4. Concentration of solutions :-

  • Dilute solution show high intensity fluroscence because in dilute solution concentration of sample is equal distrbuted.
  • In concentrated solution not give high intensity because upper layer of solution are absorbed more radiation and lower layer absorbed less radiation.

5. Oxygen :– oxygen is decreased fluroscence.

6. Affecting of PH :-

The effect of PH depends on the chemical structure of the molecule

  • Aniline in netural or alkaline medium give visible fluroscence but in acidic condition give fluroscence in uv region only
  • Phenol in acidic condition are undissociated and do not give fluroscence , but in alkaline condition they are dissociation and give good flourescent.

7. Effect of temperature :-

Increase in temperature are increase fluorescence and decrease in temperature decrease fluorescence.

Quenching :-

Quenching is the decrease in fluorescence intensity due to specific effect of constituents of the solution itself.


It is refer’s to any process which decrease the fluorescence intensity of a given substance.

Type of Quenching :-

  • 1. Chemical Quenching
  • 2. Collosion Quenching
  • 3. Self / concentration Quenching
  • 4. Static Quenching

1. Chemical Quenching :– It is further classified into four types

A. PH :- Aniline at PH 5to13 give blue fluroscence when excited 290 nm. But at PH <5 and >13 it does not exhibit fluroscence.

B. Oxygen :- Presence of oxygen lead to Quenching because of its paramagnetic property.

C. Halide and electron withdrawing group :- Halide like chloride , bromide,iodide, and electron withdrawing group like nitro and carboxylic group lead to Quenching.

D. Heavy metal :- presence of heavy metal also lead to Quenching.

2. Static Quenching :– It is due to because of complex formation.

Eg. Caffeine reduced fluorescent intensity of riboflavin.

3. Collision Quenching :-

It is due to the several factor like presence of halide, heavy metal, increase temp.,and decreased in viscosity, where number of collision are increased.

4.  Self Quenching :-

It can be due to

A. Loss in vibrational energy due to collision between atoms and molecules of a compound.

B. The emitted fluroscence radiation can be reabsorbed by the sample itself.


1. If temperature Is increased ?

A. Fluorescence is decrease

B. Fluorescence is increase

C.A and B

D. None of this

Ans. B

2. Which compound shows fluorescence ?

A. Saturated compound

B. Unsaturated molecules

C. A and B

D. None of this

Ans. B

3. Which molecules is decrease fluorescence ?

A. Oxygen

B. Hydrogen

C. A and B

D. None of this

Ans. A

4.  IF Molecular weight is increase then what happen with fluorescence

A. fluorescence increase

B. No effect on fluorescence

C. A and B

D. fluorescence decrease

Ans. D

5. Which molecule is produced Quenching ?

A. Oxygen

B. Hydrogen




6. Static Quenching is due to

A. Neutral reaction

B. Hydrogenated

C. Complex formation

D. All of the above

Ans. C

7. At which PH aniline exhibit fluorescence?

A. 1-3

B. 3-5

C. 6-8

D. 5 -13

Ans. D

8. Which type of solutions give good fluorescence ?

A. Diluted

B. Concentrated

C. A and B

D. None of this

Ans. A

9. In concentrated solution which layer is absorbed more radiation

A. Upper layer

B. Lower layer

C. A and B

D. Middle layer

Ans. A

10. Decrease in fluorescence intensity due to specific effect of constituents of the solution itself, it is called

A. fluorescence

B. Phosphorus

C. Quenching

D. Hydration

Ans c

Reference: text book of pharmaceutical analysis thrid edition of the Dr. S. Ravisankar

Page no. 3.5- 3.10

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