PHOTOCHEMISTRY

Photochemistry is the study of reaction of molecules by absorbing radiation from the electromagnetic spectrum in the U.V and visible region and becoming electronically active. The photo physical phenomenon are fluorescence and phosphorescence.

Fluorescence
When a compound absorbs light radiation, its electrons get excited to a higher energy level. The excited electrons comes to the ground state either directly or in steps with the emission of light energy. When this emission of light is instantaneous (1o^-8 sec) , the phenomenon is known as fluorescence.
i.e; When an illuminating system emits light of wavelength different from that of absorbed light, the phenomenon is known as fluorescence and it takes place as soon as the light is absorbed and ceases as soon as the light is stopped.

• Molecules having conjugated double bond or pi-bonds give fluorescence.
• Some electron donating groups such as -OH, -NH2 etc enhances fluorescence.
• Electron withdrawing groups such as -COOH, -NO etc decreases fluorescence.
• Carboxylic groups and aromatic rings decreases fluorescence.
• Certain organic chelating agents increases fluorescence.
• Polarity of the solvent also affects fluorescence.
• As viscosity of the solvent increases, the fluorescence decreases.
• Neutral or alkaline solution of aniline exhibits fluorescence in the visible region, but in acid solution, fluorescence disappears in the visible region ad appears in the U.V region.

Phosphorescence.

When a compound absorbs light energy, the electron goes to a higher energy level from the ground state. The electron then returns to the normal position by the emission of light energy. When this emission of light is observed after some time (10^-3 sec), it is known as phosphorescence.This is usually shown by solid compounds and is also termed as slow fluorescence.

i.e; The molecules with relatively stable excited state may undergo transition to a metastable triplet state and after some time returns to the ground state by the emission of U.V or visible light. This phenomenon is known as phosphorescence.

• Phosphorescence is obtained nicely at room temperature.
• The polarity of the solvent affects phosphorescence.
• Only those molecules which absorbs U.V or visible light shows phosphorescence.
• When increased phosphorescence is required, compounds containing heavy atoms are usually incorporated into the solution.
• Organic compounds containing conjugated ring systems produce phosphorescence intensely.