Photochemistry
Dr. B. R. Thorat,
Department of Chemistry,
Government of Maharashtra,
Ismail Yusuf College Jogeshwari (East),
Mumai 400060
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History
In 1817 the German physicist Theodor von Grotthus recognized that in order for light to
be effective in producing a chemical change it had to be absorbed
In 1841 the American chemist John William Draper studied the reaction between moist
hydrogen and chlorine gases. This reaction was first recognized photochemical reaction
After initiation of reaction, the rate of the reaction was proportional to the
intensity of the light absorbed
First law of photochemistry
(the Grotthus-Draper law
the amount of photochemical reaction is proportional to the quantity of light absorbed
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Quantitative Quantum theory by Max Planck in 1900 and its elucidation by
aspects Albert Einstein in 1905
Ultra violet
Initiated by
Photochemical reactions absorbing or visible
light
Wavelength range of about 800-100 nm
corresponding to energies of 36-286 kcal mole -l
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UV or visible Photon
Ground state
Organic molecule
Formed by absorption of radiation
directly or indirectly
photochemical
reaction Excited state
Product
The Stark–Einstein law of photochemistry
Absorption of one photon causes the reaction of one molecule
However, the number of molecules that react is not necessarily equal to the number of photons absorbed.
Some of the excited molecules might undergo internal conversion, intersystem crossing,
fluorescence, or phosphorescence processes leading to unreactive states and therefore not react
chemically. A chain reaction might occur in which the reaction of one molecule can lead to the
reaction of other molecules without absorption of additional radiation.
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Photochemical reactions
Stark–Einstein law of Quantum yield
photochemistry
One mole of photons is called an
Einstein
>1
For chain Non-chain
reaction reaction, <1
>1
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EXAMPLE
Upon radiation with ultraviolet light of 300 nm to 350 nm wavelength, benzophenone
undergoes a reaction with 2-propanol to form benzpinacol and acetone
Absorption of electromagnetic radiation
to produce electronically excited states
1
Photochemical
2
reactions can be
Primary photochemical reactions involving
considered to
excited electronic states, S and T
occur in three
stages
3
Secondary or dark reactions whereby the products of the primary
photochemical reaction are converted to stable products
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Terms used in photochemistry
triplet
Excited states also can have unpaired electrons
states (T)
The ground states of most molecules all The excited states also can have all
electrons are paired electrons paired
States with paired electrons are called singlet states
(S0) (S1)
Triplet state is normally are more stable than the corresponding singlet states
because, by Hund's rule, less inter-electronic repulsion is expected with unpaired
than paired electrons.
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Thermal and photochemical reactions
Thermal reactions: Thermochemistry is the study of the energy and heat associated with
chemical reactions. If a reaction releases energy and heat, it is called exothermic reaction.
The opposite is the endothermic reaction, when energy and heat is absorbed.
Photochemical reactions: Some reactions do not take place in the dark but take place only
in the presence of light or some other radiation. Such reactions are known as
‘photochemical reactions’
Thermochemical reactions Photochemical reactions
These reactions involve absorption or evolution of These reactions involve absorption of light.
heat.
They can take place even in absence of light i.e. dark. The presence of light is the primary requisite for the reaction
to take place.
Temperature has significant effect on the rate of a Temperature has very little effect on the rate of a
thermochemical reaction. photochemical reaction. Instead, the intensity of light has a
marked effect on the rate of a photochemical reaction.
The free energy change ∆G of a thermochemical The free energy change ∆G of a photochemical reaction may
reaction is always negative. not be negative.
They are accelerated by the presence of a catalyst. Some of these are initiated by the presence of a
photosensitizer. However a photosensitizer acts in a
different way than a catalyst.
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Features of photochemical reactions
Do not take place in dark but take place in the presence of light by absorbing it
The reaction which initiate by higher energy was not initiated by irradiating it with low
energy radiations while reactions which are initiated by low energy radiations are
initiated by any other high energy radiations.
For example, a photon of violet light has highest frequency and hence the highest energy.
Hence a reaction which is initiated by violet light may not be initiated by other colored
radiations of visible light. On the other hand, a photon of red light has lowest frequency
and energy. Therefore, a reaction that can be initiated by all other radiations as well.
Temperature has very little effect on the rate of a photochemical reaction. Instead, the
intensity of light has marked effect on the rate of photochemical reaction
The free energy change of a photochemical reaction may not be negative.
There are many substances which do not react directly when exposed to light. However, if
another substance is added, the photochemical reaction starts called as photo-sensitizer
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Laws of Photochemistry
Grotthus-Draper law or the first law of photochemistry: Only the light which is absorbed
by a molecule can be effective in producing photochemical changes in the molecules.
h
R R* Product
Ground state Excited state
The probability or rate of absorption is given by the Lambert-Beer law.
Where, , is called the molar extinction coefficient and is a function of frequency of
radiation, the concentration is expressed in moles per litre and l is the optical path length
in 1 cm.
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Laws of Photochemistry
Stark-Einstein law or the second law of photochemistry:
One quantum of light is absorbed per molecule of absorbing and reacting substance that
disappears or
The absorption of light by a molecule is a one-quantum process for low to moderate light
intensity.
Number of activated molecules = Number of quanta of radiation absorbed
1R + 1h 1 R*
Excited state
Ground state
Molecules which absorb photons become physically ‘excited’, and this must be
distinguished from becoming chemically ‘active’.
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Einstein energy
Chemical For any chemical reaction, energy is required in two ways: (i) as
reaction energy of activation ΔEact, and (ii) as enthalpy or heat of reaction ΔH.
On close approach, the charge clouds of the two reacting partners repel
Energy of
each other. The reactants must have sufficient energy to overcome this
activation
energy barrier for fruitful interaction.
In photochemical reactions, the energy barrier is by passed due to electronic excitation and
one of the products may appear in the excited state
The bond dissociation energy per mole for most of the molecules lie between 150 kJ and 600
kJ. These energies are available from Avogadro’s number of photons of wavelengths lying
between 800 nm and 200 nm respectively, which correspond to the visible and near
ultraviolet regions of the electromagnetic spectrum.
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Einstein energy
An Avogadro number of photons is called an Einstein
E = E2 − E1 = hν
Where, h = Planck’s constant and ν is the frequency of absorbed radiation
Wave number
Wave length
Einstein
energy
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Determination of Quantum efficiency using actinometer
Detection of the products in a photochemical
reaction
Measuring kinetics, there are more variables, since
Mechanism of we can study the effect of the intensity or the
wavelength of light on the rate of reaction
photochemical
reaction
In the detection of intermediates (S or T) by spectra
we can use the technique of flash photolysis
The use of emission (fluorescence and
phosphorescence) as well as absorption spectroscopy:
The study of quantum the presence of as well as the energy and lifetime of
yields singlet and triplet excited states
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Photochemical reactions
A reaction which takes place by absorption of the visible and ultraviolet radiations is
called a photochemical reaction
Occurs after absorption
in 10−12–10−6 s
first-order
photochemical reaction
is about 1016 s−1
intersystem crossing (ISC) in
10−12–10−4 s
Occurs in 10−16–
10−15 s
Visible/
UV
Phosphorescence
Ground
in 10−6 –10−1 s
state
As a consequence, excited triplet states are photochemically important.
Indeed, because phosphorescence decay is several orders of magnitude slower than most
typical reactions, species in excited triplet states can undergo a very large number of
collisions with other reactants before deactivation.
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Primary Secondary
Photochemical Photochemical
Photochemical Reaction
Reaction Reaction
A primary reaction proceeds by absorption of radiation. A secondary reaction is a
thermal reaction which occurs subsequent to the primary reaction
Example: The decomposition of HBr occurs as
follows
Obeys laws of
photochemistry
Thermal rea.
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Photochemical High
Low Quantum yield Quantum yield
Reaction
Deactivation of reacting molecules
Reactions subsequent to the Primary
(Collisions of fluorescence)
reaction (product of primary reaction
initiate the reaction – radical reaction,
Occurrence of reverse of primary
starting molecule is one)
reaction (polymer formation)
A reaction chain forms many molecules
Recombination of dissociated
per photon (chain reaction, two or more
fragments (fragments recombine to
starting molecules)
starting molecules)
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Causes of low quantum yield
(a) Deactivation of reacting molecules: The excited molecules in the primary process
may be deactivated before they get opportunity to react. This is caused by collisions
with some inert molecules or by fluorescence.
(b) Occurrence of reverse of primary reaction: Here the primary reaction generally yields a
polymer. The product then undergoes a thermal reaction giving back the reactant
molecules.
(c) Recombination of dissociated fragments: In a primary process, the reactant molecules
may dissociate to give smaller fragments. These fragments can recombine to give back the
reactant. Thus the secondary reactions involving the fragments to form the product will not
occur.
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Hydrogen-Chlorine reaction (high quantum yield)
A mixture of hydrogen and chlorine is exposed to light of wavelength less than 4000 Å.
The hydrogen and chlorine react rapidly to form hydrogen chloride.
In the primary step, a molecule of chlorine absorbs a photon and dissociates into two Cl
atoms. This is followed by the secondary reactions stated below:
Initiation
Propagation
The Cl atom used in step (2) is regenerated in step (3). Thus the steps (2) and (3) constitute a
self-propagating chain reaction.
This produces two molecules of HCl in each cycle. Thus one photon of light absorbed in
step (1) forms a large number of HCl molecules by repetition of the reaction sequence (2) and
(3).
The chain reaction terminates when the Cl atoms recombine at the walls of the vessel where
they lose their excess energy.
Termination
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Cause of high quantum yield
(a) Reactions subsequent to the Primary reaction: One photon absorbed in a primary
reaction dissociates one molecule of the reactant. But the excited atoms that result may
start a subsequent secondary reaction in which a further molecule is decomposed.
Obviously, one photon of radiation has decomposed two molecules, one in the primary
reaction and one in the secondary reaction. Hence the quantum yield of the overall reaction
is 2.
(b) A reaction chain forms many molecules per photon: When there are two or more
reactants, a molecule of one of them absorbs a photon and dissociates (primary reaction).
The excited atom that is produced starts a secondary reaction chain.
It is noteworthy that A consumed in (2) is regenerated in (3).
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Fluorescence: George Gabriel Stokes named the Photochemical phenomenon
phenomenon fluorescence in 1852.
Radiative decay between states of same spin
multiplicity (i.e. S=0). Emission of electromagnetic
radiation, usually visible light, caused by excitation
of atoms in a material which then reemit
immediately (within about 10-8 seconds). The
initial excitation is usually caused by the
absorption of energy from incident radiation.
Because reemission occurs so quickly the
fluorescence ceases as soon as the exciting source
is removed.
E.g. Lamp: A fluorescent light bulb is coated on the
inside with a powder and contains a gas;
electricity causes the gas to emit ultraviolet
radiation, which then stimulates the tube coating
to emit light.
Applications: Mineralogy, gemology, chemical
sensors, fluorescence labeling, dyes, biological
detectors, and, most commonly, fluorescent
lamps. The most striking examples of fluorescence
occur when the absorbed radiation is in the
ultraviolet region of the spectrum, and thus
invisible and the emitted light are in the visible
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region.
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Phosphorescence: Radiative decay between
states of different spin multiplicity (i.e. S≠
0). The emission of light from a substance Meta
exposed to radiation and persisting as an state
afterglow after the exciting radiation has
been removed. Unlike fluorescence, in
which the absorbed light is spontaneously
emitted about 10-8 seconds after excitation,
phosphorescence requires additional
excitation to produce radiation and may
last from about 10-3 seconds to days or
years depending on the circumstances.
In simple term phosphorescence is a
process in which energy absorbed by a
substance is released relatively slowly in
the form of light.
Phosphorescent substances have the ability
to store up light and release it gradually. If
the molecules of the substance can get
from the ground state to a metastable
state, and if the metastable state can Meta stable state: Particular excited state of an
slowly decay back to the ground state via atom, nucleus, or other system that has a longer
photon emission, then we have lifetime than the ordinary excited states and that
phosphorescence. generally has a shorter lifetime than the lowest,
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often stable, energy state. (shortly is energy trap)
Chemiluminescence:. It is the generation of electromagnetic radiation as light by the release of
energy from a chemical reaction. While the light can, in principle, be emitted in the ultraviolet,
visible or infrared region.
Chemiluminescent reactions can be grouped
into three types:
1. Chemical reactions using synthetic
compounds and usually involving a highly
oxidized species such as peroxide are
commonly termed chemiluminescent
reactions.
2. Light-emitting reactions arising from a living
organism, such as the firefly or jellyfish, are
commonly termed bioluminescent reactions.
3. Light-emitting reactions which take place by
the use of electrical current are designated
electrochemiluminescent reactions.
Chemiluminescence differs from fluorescence in that the electronic excited state is derived from
the product of a chemical reaction rather than the more typical way of creating electronic
excited states, namely absorption. Here, light is generated from a chemically exothermic
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Types of photochemical reactions
Photophysical process Photochemical process
the absorption of light does not the light that is absorbed by a system
result into any chemical reaction result into chemical change
fluorescence, Trommsdorff was the first person to describe the photochemical
phosphorescence reaction in 1834 - the reaction on crystals of santonin,
which when exposed to sunlight turned yellow and burst
Photosynthesis photolysis photosensitisation Photo-dissociation Photo-addition
Photo-induced rearrangements
photo-redox reaction
Photo-substitution
Photosynthesis, which sustains life on planet earth
It is a photochemical process by which green plants absorb light
energy from the sun directly and utilize it to convert atmospheric
carbon dioxide to food and oxygen in the presence of water.
Plants can convert the sunlight energy into chemical energy,
form carbohydrates using water and carbon dioxide and release
oxygen.
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Photolysis
Continuous irradiation leads to dissociation of the absorbing molecule is called photolysis
or photo fragmentation or photodissociation, or photodecomposition
The best examples for the photolysis reaction is the photolysis of aldehydes and ketones in vapour
phase. The cleavage of C-C bond (α- or β-cleavage) leads to formation of free radicals.
O O
C h C
H3CH2CH2C CH3 H3C + CH2CH2CH3
O O α-cleavage
C h C h
H3C CH3 H3C + CH3
OH
α-cleavage
C + H2C=CH2
H2C CH3
Type II cleavage β-cleavage
Diazomethane on irradiation leads to formation of unstable singlet state carbene. In the presence of high
pressure of inert gas, this species decays to triplet methylene
h N2
CH2N2 CH2 +
carbene
A type II process persists, with low quantum yield, in solution. In solution, the delay for the separation
radicals might permit the radicals to cool down and leads recombination of them
leuconitriles of triarylmethanes dissociates to ions when irradiated in polar solvents
h
Ar3CCN Ar3C + CN
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Photocatalysis
Photocatalyzed reactions are a particular class of photochemical processes that occur only in the
presence of a catalyst and upon irradiation with light
Photocatalysis was first introduced as a method for the production of hydrogen from water
(water splitting) or for the photodepollution of water or air
Commonly used photocatalysts are: Solid semiconductor particles (e. g. TiO2); Organic molecules (e. g.
benzophenone, cianoaromatics, dyes); and Inorganic salts (e.g. decatungstate anion)
The photocatalyst P absorbs the radiation and is chemically active in the excited state. once reached the
excited state (P*), it is able to activate (usually by hydrogen abstraction or electron transfer) the reagent (R).
The resulting intermediate (RA) then evolves to RI and, after reaction with the deactivated photocatalyst
(PD), gives the end product with the concomitant regeneration of P.
R-H
P*
Reagent
P* R
h
h RA EWG
P-H
PD P
P R EWG
Rl
R EWG
Product [P = (n-Bu4N)4W10O32
H
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Photosensitized reactions
An electronically excited molecule can transfer its energy to a second species which then
undergoes a photochemical process even though it was not itself directly excited
Examples: (a) Atomic photosensitizers: mercury, cadmium, zinc; (b) Molecular photosensitizers:
benzophenone, sulphur dioxide
1. Mercury acting as a photosensitizer:
2. Chlorophyll acting as a photosensitizer:
3. Chlorine photosenstizes the reaction of ozone to oxygen.
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Ozone Depletion
Troposphere,
CFCs Day to day
Activities
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Thanks
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Show that, at equilibrium stage Kp of gaseous phase chemical reaction is
Left exponentially related to ∆G of reaction.
Centre
Show that – change of A with T at constant V = change of G with T at
constant P = decrease in entropy.
Explain, partial molal quantities are intensive properties of a solution Right
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