This document discusses photochemistry, which is the study of chemical reactions caused by the absorption of light. It covers the history and definition of photochemistry, laws of photochemistry such as the Grotthuss-Draper law and Stark-Einstein law, the photochemical process involving primary and secondary reactions, types of photochemical reactions including rearrangement and substitution reactions, and applications like photosynthesis, vitamin D formation, and bioluminescence. It also discusses how photochemistry is applied to atmospheric chemistry and the formation of photochemical smog, which can have adverse health and environmental effects.

1. Photochemistry
Introduction
History
Laws of Photochemistry
Photochemical Process
Applications of photochemistry
Effects of Photochemistry
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2. Submitted to:
Dr.Areeba Farooq
Submitted by:
M.Phil. Analytical
Chemistry Semester 1st
& 2nd
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3. Participants:
Group (II) Girls
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4. Introduction
 Deals with
photochemical reactions.
 Caused by the
absorption of light
radiations from visible
and ultraviolet region.
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5. Introduction
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6. Definition of Photochemistry:
 The study of chemical reactions,
isomerization and physical behavior that may
occur under the influence of visible and/or
ultraviolet light is called Photochemistry.
 It deals with photochemical reactions caused
by the absorption of light radiations from
visible and ultraviolet region.
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7. Milestones of Photochemistry:
 1886: First organic photochemical reaction by Giacomo
Luigi Ciamician
 Early 1900’s: Grotthus- Draper law
 1905: Quantum theory of radiation (Einstein).
 1909: Wave-particle duality (Einstein)
 1912: Stark-Einstein law
 1916: Concept of spontaneous absorption and
emission (Einstein)
 1921: Nobel Prize: Einstein: Photoelectric effect
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9. Laws of Photochemistry:
 The Grotthuss-
Draper Law
 Only light that is
absorbed can produce
photochemical change
 It is the basis for
performing
photochemical and
photobiological
experiments correctly.
 The Stark-Einstein
Law
 A molecule absorbs a
single quantum of light is
becoming excited.
 For each photon of light
absorbed by a chemical
system, only one
molecule is activated for
a photochemical reaction.
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10. Laws of Photochemistry:
 The Grotthuss-
Draper Law
 If light of a particular
wavelength is not
absorbed by a system,
no photochemistry will
occur, and no
photobiological effects
will be observed, no
matter how long one
irradiates with that
wavelength of light.
 The Stark-Einstein
Law
 This law is true for
ordinary light intensities,
however, with high-
powered lasers, two-
photon reactions can
occur, i.e., the molecule
is raised to a higher
energy state than
produced by single-
photon absorption.
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11. Photochemical Process:
 All the photochemical reactions proceed at
the two stages.
1.Primary process:
 The reacting molecules undergo activation by
absorption of light.
2.Secondary process:
 The activated molecules undergo
photochemical change.
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12. Photochemical Process:
Physical Quenchig:
 Loss of energy to another molecule or atom (M)
by physical quenching, followed by dissipation of
the energy as heat
O2* + M O2 + M
Dissociation Reaction:
 Dissociation of the excited molecule (the process
responsible for the predominance of atomic
oxygen in the upper atmosphere)
O2* O + O
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13. Direct Reaction:
 Direct reaction with another species without
any intermediate product.
O2* + O3 2O2+ O
Luminescence
 Luminescence consisting of loss of energy by
the emission of electromagnetic radiation
NO2
* NO2 + hν
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14. Chemiluminescence:
 When the excited species (such as NO2 *
below) is formed by a chemical process:
O3 + NO NO2
* + O2
Intermolecular Energy Transfer:
 Excited species transfers energy to another
species which then becomes excited.
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15. PhotoIonization:
 Photoionization through loss of an electron
N2
* N2
+ + e-
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16. Types of Photochemical
Reactions:
 Rearrangement reactions (substituents move
from one atom to another in the same
molecule)
 Addition reaction (combination of molecules to
form larger molecules with no side products)
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17. Types of Photochemical
Reactions:
 Polymerization reactions (combination of
molecules to form larger molecules that contain
repeating structural units)
 Substitution reactions( replacement of functional
groups in a chemical compound with another
functional groups)
 CH4 + Cl2 CHCl3 + HCl
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18. Types of Photochemical
Reactions:
 Combination reactions ( combination of two or
more reactants to synthesize single product)
A + B C
 Decomposition reactions (splitting of single
chemical entity into two or more fragments)
AB A + B
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19. 1. Photosynthesis:
Plants use solar energy to convert carbon dioxide and
water into glucose and oxygen.
Applications of Photochemistry:
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20. The carbohydrates so form have been forming the basis
of life on earth.
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22. 2. Formation of Vitamin D
 Human rely on photochemistry for the formation
of vitamin D.
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23. 3.Bioluminescence:
 Bios – Living and Lumen – Light.
 It is a naturally occurring form of chemi-
luminescence where energy is released by a
chemical reaction in the form of light
emission.
 Production and emission of light by living
organism is called bioluminescence
 In fireflies, an enzyme in the abdomen
catalyzes a reaction that produced light.
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24. Bioluminescence:
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25. Applications of Photochemistry:
4. Vision is initiated by a photochemical reaction
of rhodopsin.
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26. 5. Ozone Formation:
 Mechanism of photochemistry also involve in
ozone formation.
 Ozone is the important species in the stratosphere,
acting as the protective radiation shield for living
organisms on earth.
 The maximum ozone concentration is around
10ppm in the stratosphere at an altitude of 25-
30km.
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27.  Ozone is formed by the photochemical reaction,
followed by:
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28. 8.Carbon Cycle:
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29. How it is applied to
atmosphere?
 In the atmosphere primary pollutants are
involved in different reactions which lead to
secondary pollutants.
 Photochemical reactions in the atmosphere -
a source of secondary pollutants like Ozone,
Acid rain, etc.
 These reactions occur in the atmosphere and
they generate photochemical smog.
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30. Photochemical Smog:
 Photochemical smog is a type
of smog produced when ultraviolet light from
the sun reacts with nitrogen oxides in
the atmosphere.
 It is visible as a brown haze, and is most
prominent during the morning and afternoon,
especially in densely populated, warm cities.
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32. Effects of Photochemistry:
 Photochemical smog has many adverse effects.
 When combined with hydrocarbons, the chemicals
contained within it form molecules that cause eye
irritation.
 Radicals in the air interfere with the nitrogen cycle
by preventing the destruction of ground level
ozone.
 Other effects include reduced visibility and
respiratory ailments.
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