Photochemistry is topic of physical chemistry bsc 3rd year

1. TOPIC :
• Photochemistry

2. What Is Photochemistry ?
• Photochemistry is the branch of chemistry concerned with the chemical
effects of light.
• Generally, this term is used to describe a chemical reaction caused by
absorption of ultraviolet (wavelength from 100 to 400 nm), visible light
(400–750 nm) or infrared radiation (750–2500 nm).
• In nature, photochemistry is of immense importance as it is the basis of
photosynthesis, vision, and the formation of vitamin D with sunlight.

3. • Photochemical reactions proceed differently to
other techniques, providing access to unique
chemistry that is both cleaner and greener.

4. Principles Of Photochemistry :
• Photochemistry is concerned with the absorption, excitation and emission of
photons by atoms, atomic ions, molecules, molecular ions, etc.
• Photochemistry is the study of the interactions between atoms, molecules, and light.
The absorption and subsequent emission of a photon by a gas phase atom such as
sodium.
• If there are two molecules in close proximity, one with an absorption band at a
wavelength shifted to the longer wavelengths than another, light energy absorbed by
the one absorbing at the shorter wavelength is usually transferred to the one that
absorbs at the longer wavelengths.

5. Examples Of Photochemistry :
• Photosynthesis: plants use solar energy to convert carbon dioxide and water
into glucose and oxygen.
• Human formation of vitamin D by exposure to sunlight.
• Bioluminescence: e.g. In fireflies, an enzyme in the abdomen catalyzes a
reaction that produced light.
• Polymerizations started by photoinitiators, which decompose upon absorbing
light to produce the free radicals for radical polymerization.

6. Concepts Of Photochemistry :
• 1. Grotthuss–Draper law and Stark–Einstein law :
• Photoexcitation is the first step in a photochemical process where the reactant is
elevated to a state of higher energy, an excited state. The first law of
photochemistry, known as the Grotthuss–Draper law (for chemists Theodor
Grotthuss and John W. Draper), states that light must be absorbed by a chemical
substance in order for a photochemical reaction to take place. According to the
second law of photochemistry, known as the Stark–Einstein law (for physicists
Johannes Stark and Albert Einstein), for each photon of light absorbed by a
chemical system, no more than one molecule is activated for a photochemical
reaction, as defined by the quantum yield

7. Kasha’s Rule :
Kasha’s rule stipulates that higher singlet states would quickly relax by
radiationless decay or internal conversion (IC) to S1. Thus, S1 is usually, but not
always, the only relevant singlet excited state. This excited state S1 can further
relax to S0 by IC, but also by an allowed radiative transition from S1 to S0 that
emits a photon; this process is called fluorescence.,

8. • Photochemical reactions require a light source that emits wavelengths
corresponding to an electronic transition in the reactant. In the early experiments
(and in everyday life), sunlight was the light source, although it is polychromatic.
• Mercury-vapor lamps are more common in the laboratory. Low pressure mercury
vapor lamps mainly emit at 254 nm. For polychromatic sources, wavelength ranges
can be selected using filters. Alternatively, laser beams are usually monochromatic
(although two or more wavelengths can be obtained using nonlinear optics) and
LEDs have a relatively narrowband that can be efficiently used, as well as Rayonet
lamps, to get approximately monochromatic beams.
Photo Chemical Reactions:

9. Schlenk tube containing slurry of orange crystals of
Fe2(CO)9 in acetic acid after its photochemical
synthesis from Fe(CO)5.

10. Fundamental Law Of Photochemical
Reactions :

11. Phenomenogical Law Of Absorption :

12. Beer – Lambert Law :

13. Kubelka & Kubelka-Munk Equations :
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