Basic photochemistry

Basic Principles
of
Photochemistry
Prof. Harish Chopra,
Department of Chemistry,
SLIET, Longowal (Pb) INDIA

Introduction
The photochemistry is the interaction of light with matter.
IUPAC has defined it as, “The branch of chemistry concerned
with the chemical effects of light (far UV to IR)”.
2
The simplest photochemical process
is seen with the absorption and
subsequent emission of a photon by a
gas phase atom such as sodium.
When the sodium atom absorbs a
photon it is said to be excited. After
a short period of time, the excited
state sodium atom emits a photon of
589 nm light and falls back to the
ground state

Importance
The photochemistry is very important as life itself depends on
photochemical processes like photosynthesis. Photochemistry
also determines the composition of Earth’s atmosphere supports
life and shields us from damaging UV radiation. Further,
photochemistry is a central branch from which many other
processes find applications.
3

Terminology
❏ CHARGE-TRANSFER (CT) TRANSITION: An electronic transition in which a
large fraction of an electron charge is transferred from one region of a molecular
entity, called the electron donor, to another, called the electron acceptor
(intramolecular CT) or from one molecular entity to another (intermolecular
CT).
❏ MULTIPLICITY (Spin Multiplicity): The number of possible orientations,
calculated as 2S + 1, of the spin angular momentum corresponding to a
given total spin quantum number (S), for the same spatial electronic wave-
function. A state of singlet multiplicity has S = 0 and 2S + 1 = 1. A doublet state
has S = ½ and 2S + 1 = 2.
❏ ELECTRONIC ENERGY MIGRATION (or Hopping): The movement of
electronic excitation energy from one molecular entity to another of the
same species, or from one part of a molecular entity to another part of the
same entity. The migration can happen via radiative or radiationless processes
4

Terminology
HUND’s RULES
❏ Of the different multiplets resulting from different configurations of
electrons in degenerate orbitals of an atom those with greatest
multiplicity have the lowest energy (multiplicity rule).
❏ Among multiplets having the same multiplicity, the lowest-energy one
is that with the largest total orbital angular momentum (angular
momentum rule) (valid if the total orbital angular momentum is a constant
of motion).
❏ In configurations containing shells less than half full of electrons, the
term having the lowest total angular momentum J lies lowest in energy,
whereas in those with shells more than half filled, the term having the
largest value of J lies lowest (fine structure rule).
5

Terminology
❏ INTERNAL CONVERSION: A photo-physical process. Iso-energetic radiation-
less transition between two electronic states of the same multiplicity. When the
transition results in a vibrationally excited molecular entity in the lower
electronic state, this usually undergoes deactivation to its lowest vibrational
level, provided the final state is not unstable to dissociation
❏ QUENCHING: The deactivation of an excited molecular entity intermolecularly
by an external environmental influence (such as a quencher) or intramolecularly
by a substituent through a non-radiative process.
❏ SELECTION RULE: A selection rule states whether a given transition is allowed
or forbidden, on the basis of the symmetry or spin of the wave-functions of the
initial and final states.
6

Laws of Photochemistry
Grothus-Draper Law (Ist Law of Photochemistry)
“When light is incident on a cell containing a reaction mixture, some portion
of the light is absorbed while the other remaining part is transmitted.
The photochemical reaction is produced by the absorbed light and
transmitted light is not effective in any way for the photochemical
transformation.
Stark-Einstein law (2nd Law of Photochemistry)
It states that each molecule involved in a photochemical reaction absorbs
only one quantum of the radiation that causes the reaction
7

Quantum Yield (ჶ)
As per IUPAC, quantum yield is the number of defined events which occur
per photon absorbed by the system. The integral quantum yield is:
(ჶ) = (number of events)/ (number of photons absorbed)
For a photochemical reaction, quantum yield can be defined as the
number of molecules of the reactant consumed or number of molecules of
the product formed per quantum of light absorbed. It is denoted by ჶ and
is also known as quantum efficiency. The value of quantum efficiency of a
reaction may vary from about zero to 106 depending upon the reaction.
8

Jablonski Diagram
9
Jablonski diagrams are frequently
used and are actually state diagrams
in which molecular electronic states,
represented by horizontal lines
displaced vertically to indicate
relative energies, are grouped
according to multiplicity into
horizontally displaced columns.
Excitation and relaxation processes
that interconvert states are indicated
in the diagram by arrows. Radiative
transitions are generally indicated
with straight arrows, while
radiationless transitions are
generally indicated with wavy
arrows.

Jablonski Diagram
10
Relaxation Mechanism for Excited State Molecules
Once a molecule has absorbed energy in the form of electromagnetic
radiation it goes to higher energy level (excited state) from ground state
as arrow upward pointing S0 S1. There are a number of routes by
which it can return to ground state.
If the photon emission (S1 S0) occurs between states of the same spin
state this is termed as fluorescence.
If the spin state of the initial and final energy levels is different (T1 S0),
the emission (loss of energy) is called phosphorescence.

Franck-Condon Principle
11
The nuclei are enormously heavy as
compared to the electrons so, during light
absorption (which occurs in femtoseconds)
electrons can move, not the nuclei. The
much heavier atomic nuclei have no time to
readjust themselves during the absorption
act, but have to do it after it is over, and this
readjustment brings them into vibrations.
Hence, Franck-Condon principle states that,
“As electrons move much more rapidly as
compared to the nuclei, so it is approximated
that in an electronic transition, the nuclei do
not change their position”.

Franck-Condon Principle
12
Fluorescence, originates from near the bottom of the upper potential curve, until it
strikes the lower potential curve. Again, it does not hit it in its deepest point, so that
some excitation energy becomes converted into vibrational energy. The
absorption-emission cycle therefore contains two periods of energy dissipation.
Because of this, the fluorescence arrow (F) is always shorter (that is, the fluorescence
frequency is lower) than that of absorption (A). In other words, the wavelengths of the
fluorescence band are longer than of the absorption band. This displacement of
fluorescence bands towards the longer wavelengths compared to the absorption bands
is called as Stokes' shift. The Stokes shift is thus displacement of fluorescence band
compared to the absorption band of a molecule.

Photochemical
Energy
13
❏ The energy of photons is dependent upon the wavelength of the light.
❏ Longer wavelength light has low energy and shorter wavelength
light has high energy.
❏ Photochemistry involves radiation between 2000 nm (near
infrared) and <100 nm (soft x-ray).
❏ The most important regions for photochemistry are:
700-400 nm (visible),
400-200 nm (Ultraviolet)
200-100 nm (Ultraviolet- visible)
❏ The energy range for photochemical dissociation (~ 170-1200 kJ
mol-1)

Chemistry of Photochemical Excitation
15
In photochemistry, the light is absorbed in the wavelength range of 200-800 nm
of the spectrum. Such a spectrum measures amount of incident light by the
molecule as a function of wavelength.
Lambert’s law states that when a monochromatic beam of light is allowed to pass
through a transparent medium, the rate of decrease in intensity with the thickness
of the medium is directly proportional to the intensity of the incident light.
Beer’s Law states that when a monochromatic light is passed through a solution
the rate of decrease in intensity with the thickness of the solution is directly
proportional to the intensity of the incident light as well as the concentration of the
solution.

Chemistry of Photochemical Excitation
16
The expression in the following equation is termed as mathematical
statement for Beer-Lambert’s law,
The quantity log (I0/It) is called as absorbance (A). The following is the
relation between absorbance (A), transmittance (T) and molar absorption
coefficient (e):
.

Electronic Transitions
17
The absorption of UV or visible radiation
corresponds to the excitation of outer electrons.
There are 3 types of electronic transition which
can be considered;
(i) Transitions involving 𝝅, 𝛔, and n electrons
(ii) Transitions involving charge-transfer electrons
(iii) Transitions involving d and f electrons
When an atom or molecule absorbs energy,
electrons are promoted from their ground state to
an excited state. In a molecule, the atoms can
rotate and vibrate with respect to each other.
These vibrations and rotations also have
discrete energy levels, which can be considered
as being packed on top of each electronic level .

18
The Absorption of ultraviolet and visible radiation in organic molecules is
restricted to certain functional groups (chromophores) that contain valence
electrons of low excitation energy. The electronic transition is involved in
promotion of an electron from one of three ground states ( 𝝅, 𝛔 or n) to one of the
antibonding ( 𝝅*, 𝛔*) molecular orbital. Organic molecules show four important
types of transitions:

19
Most absorption spectroscopy of organic compounds is based on transitions
of n or 𝝅 electrons to the 𝝅 * excited state. This is because the absorption
peaks for these transitions fall in an experimentally convenient region of
the spectrum (200 - 700 nm). These transitions need an unsaturated group
in the molecule to provide the 𝝅 electrons.

Photochemical Reaction Processes
20
A photo-excited species, [X-Y]*, can relax (react) via a
variety of pathways as given below.

21
Dissociation: The excited state species may fragment to a pair of
radicals (halogens) or, in the case of nitrogen dioxide to nitric oxide
and oxene.
2- Pentanone Propanone

22
Direct Reaction: The photo-excited state may undergo reactions
unavailable to the ground state species. E.g., a photo-excited ketone can
undergo a [2+2] cycloaddition with an alkene to give an oxetane, a
Paterno-Büchi reaction .

23
Isomerization: Photo-excited species may undergo isomerization.
E.g. trans-stilbene can be photo-excited to a state that allows free rotation
around the alkene double bond. This photo-excited species is able to relax
back to the ground state to give cis-stilbene.

24
Energy transfer: An excited state species can transfer energy to another
ground state species. This process is used to produce singlet oxygen and energy
transfer in such cases is intermolecular. A dye, usually rose bengal, is photo-
excited with UV light to transfer energy to triplet oxygen, which is converted to
singlet oxygen.
Intramolecular Energy Transfer
Intermolecular Energy Transfer

25
Quenching: In the liquid state, the
excited state species may be quenched
in which the energy of the excited state
species is converted into vibrational
energy (heat). The quenching is
generally affected with solvents .
Photoionization: These processes
are of great importance high in the
atmosphere where pressures are low
and short wavelength UV radiation
from the sun has a high flux. E.g., photo-
ionization of nitric oxide by photon
having wavelength 134.3 nm.

Selection Rules for Electronic Transitions
26
Spin Based Selection Rules Symmetry Based Selection
Rules
Transitions between electronic states
of same spin multiplicity are
ALLOWED.
Singlet Singlet
Triplet Triplet
Transitions between electronic states
of different spin multiplicity are
FORBIDDEN.
Singlet Triplet
Triplet Singlet
Transitions between orbitals of same
symmetry are ALLOWED.
= ALLOWED transition
Transitions between orbitals of different
symmetry are FORBIDDEN.
=
FORBIDDEN transition

Primary Photochemical Reactions
27
Reactions initiated by 𝛑 𝛑 * state
[S1 state]
Reactions involving Carbocations
Reactions involving Carbanions
Concerted Pericyclic reactions
Electron Transfer reactions
Cis-trans Isomerization reactions
Homolytic Fragmentation
Hydrogen atom Abstraction
Addition to Unsaturated Bonds
Rearrangement of stable carbon
centred radicals
Reactions initiated by n 𝛑 * state
[T1 state]

Photosensitized Reactions
28
Reactants in some chemical reactions do not absorb light and no product
is formed on exposure of such reactants to radiations. However, if with
the addition of another substance, which can absorb radiations, these
reactants are converted into products. In actual the substance added
absorbs light and becomes excited and then passes this energy to one of
the reactants, which gets activated to react with the other reactant(s) to
give products. The substance which when added to a reaction mixture to
help in initiating a photochemical reaction without undergoing any
chemical change is called a photo-sensitizer and these types of reactions are
called photosensitization reactions. The process is called as
photosensitization. The most commonly used photosensitizers include
mercury, cadmium, benzophenone and sulphur dioxide.

Mechanism of Photosensitization
29
A general donor-acceptor system in which donor
(sensitizer) absorbs the incident light and becomes
excited. The triple state of the donor is higher than the
triple state of the acceptor that is reactant. On
absorption of photon donor changes to singlet excited
state and then it changes to triplet excited state by
intersystem crossing (ISC). This triplet state then collides
with acceptor producing triplet excited state of acceptor (T1)
and ground state of donor, when triplet state of acceptor
gives the desired product then the mechanism is called
photosensitization. The triplet excited state of sensitizer
must be higher in energy than triplet excited state of the
reactant so the energy available is sufficient to raise the
reactant molecule to its triplet excited state.
.

Applications of Photosensitization
30
Isomerization of but-2-ene:
SO2 acts as photosensitizer in this
isomerization reaction. The cis-but-2-ene
and SO2 vapour are irradiated with light
of 𝞴 = 254 nm leading to the formation of
trans-but-2-ene
Dimerization of Cyclohexane:
Hg is used as photosensitizer in this
reaction. The mixture of cyclohexane and
mercury vapour are irradiated with light
of 𝞴=254 nm to give dimerized products

31
References
❏ Chemistry For Engineers, Harish Kumar and Anupama
Parmar, ISBN No. 978-81-8487-545-4 (2016), Published by
Narosa Publishing House Pvt. Ltd., New Delhi. .
❏ NPTEL Lectures & Videos
❏ Internet sources

Basic photochemistry

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