Introduction to photochemistry

1. Dr. Anshul Bansal
Assistant Professor
S. A. Jain (PG) College
Ambala City
Photochemistry
1Dr Anshul Bansal

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Photochemistry is the study of the
interaction of electromagnetic radiations
with matter resulting into a physical change
or a chemical reaction.
The visible and UV radiations (200-800 nm
wavelength) are mainly used in photochemical
reactions. Far UV radiations are used for water
purification and lab equipment sterilization. UV
radiations are used in drug detection and printer inks.
Visible radiation helps us see and perform visible
spectral studies.
Dr Anshul Bansal

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Essential criteria for all photochemical
reactions
Molecule must absorb light for a photochemical or
photophysical change to occur.
Ultraviolet and visible radiation interacts with
matter which causes electronic transitions
(promotion of electrons from the ground state to a
high energy state).
Radiation energy must match energy difference of
ground and excited state.
Dr Anshul Bansal

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Processes studies under photochemistry
1. Photophysical processes
Those processes which take place in the presence of light but
do not result into any chemical reaction.
Examples are fluorescence, phosphorescence and photoelectric
effect.
2. Photochemical processes or photochemical reactions
Those reactions which take place by absorption of light by the
reacting substances.
Reactions are brought about by the absorption of light
radiations of the Visible and UV region (800-200 nm).
Examples
i) Photosynthesis of carbohydrates in plants.
ii) Combination of H2 and Cl2 to form HCl
Dr Anshul Bansal

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Difference between photochemical and
thermochemical reactions
Dr Anshul Bansal

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Laws governing absorption of light
Lambert’s Law: This law states that decrease in the intensity of
monochromatic light with the thickness of the absorbing medium is
proportional to the intensity of incident light.
-dI/dx = kI
where dI is small decrease in intensity of light on passing through small
thickness dx, I is intensity of incident light before entering thickness dx
and k is constant of proportionality called absorption coefficient depending
upon nature of absorbing medium
Beer’s Law: It states that decrease in the intensity of monochromatic light
with the thickness of the solution is not only proportional to the intensity of
the incident light but also to the concentration (c) of the solution.
-dI/dx = eIc
where e is constant of proportionality called molar absorption coefficient. Its
value depends upon the nature of absorbing medium and wavelength of light
used.
Dr Anshul Bansal

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Some commonly used terms
Transmittance (T): The ratio of intensity (I) of the transmitted light from the
solution to the intensity of the incident light (I0) i.e. the ratio I/I0
T = I/I0
Absorbance (A) or Optical density (D) or Extinction (E): The logarithm of
the reciprocal of transmittance i.e. log 1/T or log I0/I is called absorbance A.
A = E = D = log 1/T = log I0/I
Specific extinction coefficient (Ks): It is defined as the extinction or optical
density or absorbance per unit length and unit concentration of the solution.
K s = D/cx
Molar extinction coefficient (eꞌ):
-log I/I 0 = -log T = A (or D) = eꞌcx
If x = 1 cm and c = 1 mol L-1 then eꞌ = A or D
Hence, molar extinction coefficient may be defined as the optical density or
absorbance of the solution having 1 cm thickness and concentration equal to 1
mol L-1
Dr Anshul Bansal

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Laws of photochemistry
1. Grotthus-Draper Law or First law of photochemistry: When light falls
on a body, a part of it is reflected, a part of it is transmitted and rest of it is
absorbed. It is only the absorbed light which is effective in bringing out
photochemical changes in the molecule.
The above law, however does not imply that the absorbed light must
always result into chemical reaction.
2. Stark-Einstein’s Law of photochemical equivalence or Second law of
photochemistry: Every atom or molecule that takes part in a
photochemical reaction absorbs one quantum of radiation to which the
substance is exposed.
Energy absorbed by one mole of the reacting molecules will be given by:
E = Nh
where N = Avogadro’s number, h = Planck’s constant and  = frequency
of absorbed radiation.
The energy possessed by one mole of photons is called one einstein.
Dr Anshul Bansal

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Value of einstein in different units
In CGS units
N = 6.022 x 1023 mol-1
h = 6.626 x 10-27 erg sec and c = 3.0 x 1010 cm/sec
E = 2.86/(cm) cal per mole
where  is in cm but generally  is usually expressed in Angstrom (Å) and
we know that 1 Å = 10-8 cm thus we get
E = 2.86/(Å) x 108 cal per mole
Or E = 2.86/(Å) x 105 Kcal per mole
In SI units
N = 6.022 x 1023 mol-1
h = 6.626 x 10-34 Js and c = 3.0 x 108 m/sec
E = 0.1197/(m) J mol-1
Or E = 11.97 x 10-5/(m) kJ mol-1
Also remember, 1 eV = 1.602 x 10-19 J = 96.48 kJ mol-1 = 23.06 kcal mol-1
Dr Anshul Bansal

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Quantum yield or quantum efficiency (f)
ф =
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑒𝑠 𝑟𝑒𝑎𝑐𝑡𝑖𝑛𝑔 𝑖𝑛 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑡𝑖𝑚𝑒
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑞𝑢𝑎𝑛𝑡𝑎 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑡𝑖𝑚𝑒
ф =
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑚𝑜𝑙𝑒 𝑠 𝑟𝑒𝑎𝑐𝑡𝑖𝑛𝑔 𝑖𝑛 𝑎 𝑔𝑖𝑣𝑒𝑛 𝑡𝑖𝑚𝑒
𝑁𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑒𝑖𝑛𝑠𝑡𝑒𝑖𝑛𝑠 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑡𝑖𝑚𝑒
If a reaction obeys the Einstein law, one molecule is
decomposed per photon, the quantum yield f = 1.
When two or more molecules are decomposed per photon,
the quantum yield f >1 and the reaction has a high quantum
yield.
When the number of molecules decomposed is less than
one per photon, the quantum yield f < 1 and the reaction has
a low quantum yield.
Dr Anshul Bansal

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Thank you
Dr Anshul Bansal