Photochemical reactions are chemical reactions initiated by the absorption of light energy. When molecules absorb light, they enter transient excited states with different chemical and physical properties. Chain reactions involve reactive intermediates that propagate the reaction by inducing additional reactions. In 1913, Max Bodenstein proposed the idea of chemical chain reactions, and in 1918 Walther Nernst suggested that the reaction of hydrogen and chlorine occurs by a chain mechanism involving reactive radical intermediates.

2. Photochemical reaction,
a chemical reaction initiated by the
absorption of energy in the form of
light. The consequence of molecules’
absorbing light is the creation of
transient excited states whose
chemical and physical properties
differ greatly from the original
molecules

3. chain reaction:
A chain reaction is a sequence of reactions where
a reactive product or by-product causes additional
reactions to take place.

4. History
In 1913, the German chemist Max Bodenstein first put forth the idea of
chemical chain reactions. If two molecules react, not only molecules of the final
reaction products are formed, but also some unstable molecules which can
further react with the parent molecules with a far larger probability than the
initial reactants
 In 1918, Walther Nernst proposed that the photochemical reaction between
hydrogen and chlorine is a chain reaction in order to explain what's known as
the quantum yield phenomena. This means that one photon of light is
responsible for the formation of as many as 106 molecules of the product HCl

5. Chain reactions
 A quantitative chain chemical reaction theory was created later on by Soviet
physicist Nicolay Semyonov in 1934. Semyonov shared the Nobel Prize in
1956 with Sir Cyril Norman Hinshelwood, who independently developed
many of the same quantitative concepts.
 Typical steps:
 Initiation Propagation Termination

6. Hydrogen bromine system
 Initiation (formation of active particles or chain carriers, often free radicals, in
either a thermal or a photochemical step)
 The reaction H2 + Br2 → 2 HBr proceeds by the following mechanism:[4][5]
Initiation
 Br2 → 2 Br• (thermal) or Br2 + hν → 2 Br• (photochemical)
 each Br atom is a free radical, indicated by the symbol « • » representing an
unpaired electron.

7.  Propagation
may comprise several elementary steps in a cycle, where the active particle
through reaction forms another active particle which continues the reaction
chain by entering the next elementary step
 * chain branching (a propagation step which forms more new active
particles than enter the step);
 * chain transfer (a propagation step in which the active particle is a
growing polymer chain which reacts to form an inactive polymer whose
growth is terminated and an active small particle (such as a radical), which
may then react to form a new polymer chain).

8.  Propagation (here a cycle of two steps)
 Br• + H2 → HBr + H•
 H• + Br2 → HBr + Br•
 the sum of these two steps corresponds to the overall reaction H2 +
Br2 → 2 HBr, with catalysis by Br• which participates in the first
step and is regenerated in the second step.

9. Termination
(elementary step in which the active particle loses its activity; e. g. by
recombination of two free radicals).
2 Br• → Br2
recombination of two radicals, corresponding in this example to initiation in
reverse.

10. Factors affect on photochemical reactions:
 The rate of photochemical reactions is affected by the intensity of
light.
2. Temperature has little effect on photochemical reactions.
 Light energy (uv or visible radiation) can initiate or catalyse
particular chemical reactions.
As well as acting as an electromagnetic wave, light can be considered
as an energy 'bullets' called photons and they have sufficient
'impact energy' to break chemical bonds, that is, enough energy to
overcome the activation energy.
The greater the intensity of light (visible or ultra-violet) the more
reactant molecules are likely to gain the required energy
(activation energy) and react, so the reaction speed increases.

11. Examples:
 Examples of photochemical reactions[edit]
 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.[7]
 Polymerizations started by photoinitiators, which decompose upon absorbing
light to produce the free radicals for radical polymerization..
 Photodynamic therapy: light is used to destroy tumors by the action of singlet
oxygen generated by photosensitized reactions of triplet oxygen. Typical
photosensitizers include tetraphenylporphyrin and methylene blue. The
resulting singlet oxygen is an aggressive oxidant, capable of converting C-H
bonds into C-OH groups.
 Photoresist technology, used in the production of microelectronic components.