introduction OPC

2. ❑ Vinay P. Sharma, Rakesh Kumar; Pericyclic Reactions and Organic
Photochemistry; Pragati First Edition : 2008
❑ Dwaine O. Cowan, Ronald L. Drisko; Elements of Organic
Photochemistry; Plenum Press; 1976
❑ Jacques Kagan; Organic Photochemistry: Principles and
Applications; Academic Press; 1993
❑ Biswanath Dinda; Essentials of Pericyclic and Photochemical
Reactions; Springer International Publishing Switzerland 2017
Text books

3. Organic Photochemistry
• It has become known to those who study chemistry or any of the natural sciences as well, in order to a chemical
transformation (or even physical conversions) take place, Energy is needed to initiate these changes.
• Chemistry is a branch of the natural sciences that deals with various scientific aspects of the chemical
structure and transformations of matter at the molecular level.
This required Energy (which we call Activation energy) can be supplied in two different ways
First, by increasing the temperature of
reaction mixture, this produces a
continuous increase in energy.
Secondly, by irradiating the reaction
mixture with radiations of suitable energy
which produces discrete energy gain.
• The Photochemistry is concerned with the interaction of visible and ultraviolet light with molecules,
an important aspect of modern chemistry which is relevant to biology (e.g. photosynthesis, vision), lasers,
organic synthesis, reaction kinetics and atmospheric science (e.g. the ozone hole).
The study of chemical reactions, isomerizations and physical behavior that may occur to a
molecule under the influence of visible and/or ultraviolet light is called Photochemistry.

4. Photochemical Reactions
• Photochemical reactions occur all around us, being an important feature of many chemical processes that occur in living
systems and in the environment.
• Photochemical reactions have a great impact on biology and technology, both good and bad.
• Vision in all animals is triggered by photochemical reactions.
• The destructive effects of ultraviolet radiation on all forms of life can be traced to photochemical reactions that alter
cellular DNA, and the harmful effects of overexposure to sunlight and the resulting incidence of skin cancer are well
established.
• The technical applications of photochemistry are manifold.
• The dye industry is based on the fact that many organic compounds absorb particular wavelengths of visible light, and the
search for better dyes and pigments around the turn of the last century was largely responsible for the development of
synthetic organic chemistry.
➢ At the end of the twentieth century, the power and versatility of photochemistry is becoming increasingly
important in improving the quality of our life, through:
• health care,
• energy production
• and the search for “green” binary solutions to current problems.
➢ Many industrial and technological processes rely on applications of photochemistry,
➢ and the development of many new devices has been made possible by spin-off from photochemical research.

5. • Photochemical reactions of organic compounds have attracted much interest in the
recent times for its fascinating nature and wide applications in the synthesis of
organic compounds.
➢There are two key features of photochemical reactions which give them special
importance over thermal reactions.
• First, the reactions take place in the excited state of the molecules having a large excess of
energy compared to ground state,
• it is often possible to effect reactions which are thermodynamically unfavorable due to their
ground-state reactants.
• Second, the reactions are usually carried out at low temperatures so that the products can be formed in
cold.
• Hence, it is often possible to make highly strained ring systems by pumping out excess energy as light
to overcome the activation energy barrier in their formations.

6. Photochemical and Thermal Reaction
Activation Energy
• Generally, energy is supplied to bring a chemical
transformation.
• The reacting molecule first acquire enough energy
to cross the energy barrier separating reactants and
products.
• This required amount of energy is known as
Activation energy (Ea).
• In some cases activation energy may be so low that molecules have enough thermal energy at
room temperature. Under these conditions a spontaneous reaction will occur.
• Additional energy is supplied for nonspontaneous reactions.

7. Energy Intake
• As the temperature of the system raised the
molecules moves more rapidly, i.e., translational
energy increases.
• Some of these molecules moves faster and other
slowly than the average ones.
• This additional movement facilitates more
collision.
• At the same time molecules acquires additional
rotational and vibrational energies also.
• When the energy absorbed by molecule, higher
vibrational states [V1, V2 etc] get populated and
reach excited state.
Photochemical and Thermal Reaction

8. • In typical organic moles V1 lies 2 to 10 Kcal/mol from
V0 vibrational state.
• At room temperature molecules have an thermal
energy content of about 0.6 Kcal/mol.
• Many chemical reactions especially those that are
intermolecular, involves these higher vibrational
levels.
• This energy is sufficient to rupture the bond [bond
dissociation energy].
• The bond dissociation energy varies widely and
depends upon the structure of the molecule and nature
of the atoms involved in the bond.
• About 101 Kcal/mol for C-H bond in methane and 83
Kcal/mol for C-C bond in ethane, is required bond
dissociation energy.
Photochemical and Thermal Reaction

9. • On the other hand, absorption of light provides a large amount of energy to the molecules or to
specific part of the molecule directly.
• Light of Infra-red region [> 8000 Å] having energy about 10 Kcal/mol which is sufficient for
vibrational excitation.
• Light of shorter wavelength, visible region [4000 -8000 Å], ultraviolet region [2000 - 4000 Å] and
near ultraviolet region [100 - 2000 Å] of electromagnetic spectrum corresponds to energy about 36 to
143 kcal/mol.
Photochemical and Thermal Reaction

10. • Light of UV-visible region have sufficient energy
for electronic excitation of organic molecules.
• Morse curves represents different electronic states,
lower curve represents electronic ground state [E0]
while, upper curve shows electronic excited state
[E1].
• The transition involves excitation of an electron
from a bonding molecules orbital to an antibonding
molecule orbital.
Photochemical and Thermal Reaction

11. • Each electronic state is associated with a number of
vibrational and rotational energy levels.
• So not a specific radiation frequency but a region of
electromagnetic radiation is taken into account.
• The electronic excitation is superimposed upon
rotational and vibrational levels.
• Hence, during promotion, the electron moves from a
vibrational and rotational level within one electronic
mode to some other vibrational and rotational level
of the higher electronic mode.
Photochemical and Thermal Reaction

12. • So, there are a large number of transitions possible
which are close together and responsible for change
in electronic (E), vibrational (v) and rotational (r)
energy levels.
• Hence, not just one wavelength but a large number
of wavelengths which are close enough will be
absorbed by same molecule [A region of
electromagnetic spectrum be absorbed].
Photochemical and Thermal Reaction

13. • Any organic reaction needs energy to occur,
• This Energy can be stored in the reagents/reactants in cases where very reactive or high
energy contents species are used,
• or should be supplied by other means from external sources.
Photochemical and Thermal Reaction

14. • The total energy (E) of a molecule (apart from nuclear and kinetic energy) can be
expressed as the sum of four energy terms:
E = E electronic + E vibrational + E rotational + E translational
• The first three energies are quantised, i.e., they can change only by discrete energy jump
or fall,
• while transnational energy (Energy due to molecular movement) is not quantised and can
change in a continuous manner.
Photochemical and Thermal Reaction

15. E = E electronic + E vibrational + E rotational + E translational
In usual laboratory organic chemistry,
• energy is supplied thermally or from microwaves
or ultrasounds as more recently reported.
• All these sources of energy usually affect only the
ground state of a molecule by increasing its
translational, rotational and vibrational energy,
• although under special conditions electronic
excited states can be reached or involved.
Photochemical and Thermal Reaction

16. E = E electronic + E vibrational + E rotational + E translational
• Thermal excitation introduces energy randomly into
transnational, rotational and vibrational modes, producing an
energy distribution in the system such that most molecules have
about the same amount of energy.
• Absorption of electromagnetic radiation of UV-visible region,
excites an individual molecule instantaneously to an excited
electronic state.
• This promotion involves transition of an bonding electron to an
antibonding electronic state.
• Thus, a large amount of energy is placed in a single molecule.
Photochemical and Thermal Reaction

17. • When the energy of the electromagnetic radiation falls in
the ultraviolet (UV) or visible region (VIS) which is
sufficient to promote electrons to excited states of either
atoms or molecules, photo-chemical processes can be
observed.
• In photo-chemical reactions light must be considered as a
reagent which interacts or in chemical terms “reacts”
directly with the electrons (σ and π electrons and lone
pairs).
Photochemical and Thermal Reaction

18. • In a classical thermally stimulated reaction, the extra energy supplied to the reagents, is necessary for reaching and
overcoming the transition state at higher energy and for its evolution into the final products.
• Similarly, when the light (or photons in its quantum mechanics description) interacts with the electrons of a molecule
or an atom, a high energy state is generally produced which can be pictured as the transition state of a photo-chemical
reaction.
Photochemical and Thermal Reaction

19. • This high energy state or electronically excited state of the molecule is the heart of all “photo-stimulated
processes”.
Thermal and photo-chemical stimulated processes are strongly different:
• In thermal processes all atoms of the molecule in its ground states are stimulated with a continue variation of the
energy along the reaction coordinate;
• photo-chemical process deals directly with the quantum or discrete nature of the light-matter interaction
Photochemical and Thermal Reaction

20. Photochemical Reactions
• These reactions involve absorption of light.
• The presence of light is the primary requisite for the
reaction to take place.
• Temperature has very little effect on the rate of a
photochemical reaction. Instead, the intensity of light
has a marked effect on the rate of a photochemical
reaction.
• The free energy change ΔG of a photochemical reaction
may not be negative.
• Some of these are initiated by the presence of a
photosensitizer. However a photosensitizer acts in a
different way than a catalyst.
Thermal Reactions
• These reactions involve absorption or evolution of heat.
• They can take place even in absence of light i.e. in dark.
• Temperature has significant effect on the rate of a
thermochemical reaction.
• The free energy change ΔG of a thermochemical
reaction is always negative.
• They are accelerated by the presence of a catalyst.

21. Photochemical vs Thermal Reaction
• Thermal excitation introduces energy randomly into transnational, rotational and vibrational modes, producing an
energy distribution in the system such that most molecules have about the same amount of energy.
• Absorption of electromagnetic radiation of UV-visible region, excites an individual molecule instantaneously to an
excited electronic state.
• This promotion involves transition of an bonding electron to an antibonding electronic state. Thus, a large amount of
energy is placed in a single molecule.
• Thermal and photochemical excitations provide two complementary methods for introducing energy into
molecule.
• That's why these two methods provides too different chemical sequences.
• Now we can say photochemistry is the study of the chemistry of electronically excited molecules produced by the
absorption of electromagnetic radiation