Advanced Photochemistry: Principles, Mechanisms, and Applications in Chemical Reactions

CHEMISTRY PHOTO CHEMISTRY
ADVANCED
PHOTOCHEMISTRY
COURSE NO. PC 503
By
DR. M. EMRAN QUAYUM

Photochemistry : It is the study of reactions that
are brought about by the action of visible or
ultraviolet light. e.g. photosynthesis of glucose in
the plants by means of sunlight.
6 CO2 + 6 H2O
sun light
chlorophyll
C6H12O6 + 6 H20

Chemical reactions accompanied with light are :
By the action of light
A → B
(i) A new molecule is generated by chemical changes
(light induced reactions)
(ii) In some reaction, during chemical reaction light
is emitted (chemiluminescence)

Luminescence
- Chemiluminescence:
(green)
White phosphorus glows ignites in air to form phosphorus pentoxide.
-Bioluminescence : - mushrooms
(emission of light
by a living organism) - insects
- fishes
P4(g) + O2(g) + H2O P4H10 + hv

Light : Electromagnetic field vibration spreading in
quanta (photons).
Photon: The smallest amount of light carrying
energy.
Energy of photons (A. Einstein)
E = c

h h
=
SO one particle of a chemical substance can absorb only
one photon from a light beam: E = h

Chemical bond energies: from 100 – 1000 kJ/mol
Light energies:

604 kJ/mol-1 302 151
200 nm 400 nm 800 nm
ULTRAVIOLET VISIBLE INFRARED
So UV – and VIS region is expected to induce chemical reactions.

Laws of Photochemistry
(I) Only light that is absorbed can produce photochemical
change (Grotthus, Draper)
(ii) A molecule absorbs a single quantum of light is
becoming excited (Stark, Einstein)

Basic Principles of Photochemistry :
Photochemistry is based on following
fundamentals.
1. Photochemical Energy.
2. Electronic Excitation.
3. Excited State, Modes of Dissipation of Energy.
4. Energy Transfer.
5. Quantum Efficiency.

1. Photochemical energy:
For any chemical transformation an activation energy, must be
supplied to molecules.
Such energy can be provided by different way :
(i) Some molecules undergo spontaneous transformations.
(ii) In some cases, energy is supplied by increasing the
temperature (thermal condition) as a result molecules present
in the system have same amount of energy throughout the
chemical transformation.

(iii) In another method, molecules present in the system involve
the absorption of electromagnetic radiation in the visible or
ultraviolet region (photochemical condition).
Such absorption of light excites an individual molecule from
ground state to an excited electronic state without effecting the
surrounding molecules.
- selective excitation
-chemistry of excited molecules differs from the chemistry of
those in the ground state
- One can change the course of a reaction by activating the
reactants by light rather than by heat.

Infrared region is producing Vibrationally or rotationally
excited molecules,
While light in the visible and ultraviolet region has
sufficient energy to cover the range of chemical bond energies
and is able to induce chemical changes by exciting
molecules to higher electronic states.

2. Electronic excitation:
A. B. M. O
.
B. M. O
.
A. O. Atomic Orbi tal
E

*
Promotion of an electron from the bonding orbital to the
corresponding anti-bonding orbital take place .
 - * ;  - * and n- *.

Molecular Orbital
Diagram
CH2
= CH – CH = CH2
E
Antibonding Orbital
Bonding Orbital
Ground
Sta te
Excited
State
Ψ
Ψ
Ψ
Ψ
4
3
2
1
In butadiene  - * transition occurs, from Ψ2
to Ψ3

3. Excited States, Modes of Dissipation of Energy :
In an organic molecule, even number of electrons are paired
in the ground state.
Now when absorption of light of the correct energy occurs,
than one of the electron excited from ground state () to
excited state(*) by retaining the spin, so the electron spins
remain paired in the excited state. This state is called
excited singlet state (S1).
In some cases, spin inversion take places thus giving rise to a
new excited state with two unpaired electrons. This state is
called an excited triplet state (T1).

*

Ground
State
Singlet
State
Triplet
State
Antibonding
Bonding
E
S0 S1 T1
A triplet state is more stable than the singlet state. Because
in triplet state, electrons are unpaired - lesser inter electronic
repulsion take place, while in singlet state electrons are paired, -
causing inter electronic repulsion and is unstable.

Jablonski Diagram:
10- 08 Sec.
10- 11Sec. Su
S1
S2
S0
T1
I. C.
R. D.
I. S. C.
P
F
10- 03-- 1Sec.
R. D.
li ght
10- 09-- 10- 06Sec.
light
10- 05-- 10- 03Sec.
ISC = Intersystem crossing. IC = Internal conversion.
F = Fluorescence. P = Phosphorescence.
RD = Radiationless decay.

A molecule in S1
state undergo one of the following four energy
degrading (decay) processes to the ground state.
(i)The molecule can undergo chemical reaction or return to the
ground state by emission of light by a process called fluorescence
and generally occurs with in 10-9
to 10-6
sec.
(ii) It may return to ground state (S0
) by non radiative process in
which excess energy of the excited state is shuffled into vibrational
modes.
(iii) S1
may undergo chemical reactions.
(iv) The molecule may undergo spin inversion to the triplet state
(spin unpaired) by a process as ”intersystem crossing”,
Which is a radiation less process.
When a molecule absorbs a photon of energy, electronic transition
occurs (Su or S2 or S1).

Fluorescence : Emission of a photon from a singlet excited
state to a singlet ground state, or between any two energy
levels with the same spin, is called fluorescence. Fluorescence,
decays rapidly after the excitation source is removed. Lifetime
of the electron is only 10–5
to 10–8
s
Phosphorescence : Emission between a triplet excited state
and a singlet ground state is called phosphorescence.
phosphorescence may continue for some time after removing
the excitation source. Lifetime for phosphorescence ranges from
10–4
to 104
s.

4. Energy Transfer and Photosensitization:
It is a one step radiationless transfer of excitation energy
from an electronically excited molecule (donor) to the ground
state of another molecule (acceptor).” As a result, the donor
molecule returns to the ground state and the acceptor
molecule gets excited.
For energy transfer the donor molecule should have at least
5 kcal/mole more energy than the acceptor molecule.
A typical mechanism for triplet energy (T1
) transfer is
described below :

D 1D
Ground State( S0) Singlet State ( S1)
Triplet State ( T1)
3D
A 3A
hv
n 
*
_
Singlet State ( S1)
1D
3D
Tri plet State ( T1) Ground State( S0)
+ +
Ground State( S0)
Tri plet State ( T1)
D
Triplet State ( T1)
3A
Products
Where D= Donor & A= Acceptor
1= Singlet & 3= Triplet

Energy transfer between butadiene as an acceptor and
benzophenone as donor.
Butadiene (I) (acceptor) upon direct irradiation leads to
produce product (II) and (III) via ring closure.
( I ) h v
vi a Sensitizer
( II) ( III)
( IV) ( V) ( VI)
h v
+
+ +
When butadiene is mixed with benzophenone (donor) and is
irradiated at 366 nm, than butadiene undergoes photochemical
change to yield dimmers (IV) (V) and (VI).

In this process light is absorbed by benzophenone (donor)
but the reaction is taking place with butadiene (acceptor) .
Here benzophenone (donor) is functioning as a
photosensitizer, while butadiene is good acceptor as it has 9
kcal/mole less triplet energy than benzophenone.
( C6H5) 2CO hv ISC
S0 S1 T1
D
A
Dimers
( I V + V + VI ) +
1[ ( C6H5) 2CO]
D
3[ ( C6H5) 2CO]
D
S0
D S0
( C6H5) 2CO
3
[ ]
A T1

S1
S0
T1
Benzophenone 1, 3- Butadiene
Energy Transfer
69 kcal/mole
S1
S0
T1
120 kcal/mole
60 kcal/mole
ISC
74 kcal/mole
Benzophenone (69 Kcal/mole) (donor) triplet energy transfer
to 1,3 - Butadiene (60 Kcal/mole) (acceptor) (Intermolecular
energy transfer ).

C
O
( CH2 ) n
Intramolecular energy transfer in benzophenone (donor)
and naphthalene.
Irradiation of these compounds with light of about 366 nm
wavelength absorb by benzophenone showed that there is
an efficient transfer of triplet excitation from the
benzophenone moiety to the naphthalene moiety.

energy transfer in Cis – Trans Isomers
of 4-hexen-2-one.
C
CH3
H
CH2
C
O
hv
C
H
CH3 CH3
H
C
O
C
CH2
H
CH3
C
Trans- Isomer Ci s- Isomer
_ 
*
n
During this photoreaction energy transfer from the carbonyl group
to the carbon-carbon double bond takes place.
Cis - trans isomerization of 4-hexen-2-one shows intramolecular
energy transfer when light is absorbed by the carbonyl group.

5. Quantum Efficiency.
It is the number of moles of a reactant disappearing, or the
number of moles of a product produced, per einstein of
monochromatic light absorbed
= NUMBER OF MOLECULES UNDERGOING THE REACTION
ɸ /
NUMBER OF PHOTONES ABSORBED BY THE PHOTOREACTIVE
SUBSTANCE

Molecular orbital construction of
n - * transition
C O
S0
hv
S1
O
C
n 
*
_
C O
n - * excitation of the carbonyl group.
The remaining two electrons of oxygen atom present in p-
orbital
are known as lone pair or n electrons. In n - * transition, an
electron of the lone pair of “O” is prompted to a vacant *
orbital
as shown below :

C O _
n
hv

*
O
C C O
In terms of V. B. T., the excitation may be represented as,
n - * transition is always less intense than  - * transition
even though it occurs at longer wavelength.

Photoreduction:
When a solution of benzophenone in isopropyl alcohol is
irradiated with a light of 345 nm it produce a benzpinacol.
During this photochemical reaction isopropyl alcohol does
not
absorb light but benzophenone undergoes an n - *
transition.
C6H5 C
O
2 CH3 CH
OH
hv
( C6H5) 2
C6H5 + CH3 _ 
*
n
C C ( C6H5) 2
OHOH
+ CH3COCH3
Benzophenone Benzpi nacol Acetone
Isopropyl alcohol

In benzophenone, an initial light absorption is followed by
rapid intersystem crossing from S1
to T1
.
( C6H5) 2CO
1
[ ( C6H5) 2CO] 3
[ ( C6H5) 2CO]
I. S. C
.
hv
Mechan
ism
n - * triplet state of carbonyl group of benzophenone is
capable of abstracting the α - hydrogen of isopropyl alcohol, thus
giving rise to produce two radicals A & B.

3
[ ( C6H5) 2CO] CH3
OH
CH
CH3
+ OH
C
C6H5
C6H5
CH3 C OH
CH3
+
( A) ( B)
combination of two units of A leads to form benzpinacol.
C
OH
Benzpinacol
OH
OH
( C6H5) 2
C
C
( C6H5) 2
( C6H5) 2
2
Since the quantum yield () for the formation of benzpinacol is 1,
the second unit of ‘A’ does not form as a result of absorption of a
fresh quantum of light. However, it is generated by the reaction
between benzophenone and unit B. i.e.

CH3
OH
C
CH3
C6H5
O
C
C6H5
+
Acetone
CH3 C CH3
O
( A)
C6H5
C6H5 C OH
+
Benzophenone
Similarly benzpinacol is formed during the reaction between
benzophenone and benzhydrol in benzene
( C6H5) 2CO + ( C6H5) 2CHOH
hv
Benzhydrol
Benzophenone Benzpi nacol
OH
OH
( C6H5) 2
C
C
( C6H5) 2

Mechanism:
hv I. S. C
. 3
[ ( C6H5) 2CO]
1
[ ( C6H5) 2CO]
( C6H5) 2CO
( i)
3
[ ( C6H5) 2CO] + ( C6H5) 2CHOH
C6H5
C6H5 C OH
( ii)
( iii)
OH
C
( C6H5) 2
2
2
Benzpinacol
OH
OH
( C6H5) 2
C
C
( C6H5) 2

Michler’s ketone does not undergo
photoreduction in isopropyl alcohol
E
C
O
( CH3) 2N N( CH3) 2
O
C
n 
* n 
*

*


*

In michler’s ketones, the lone pair of electrons situated on nitrogen atom is in
conjugation with the aromatic rings, which lowers the energy required for the
 - * transition in the molecule. Thus  -* transition becomes the lowest
triplet of the system and occur more rapidly than the n - * transition. The
relative energies of n - * and  - * transition are shown below :

Photoreduction of α – ketoester
α – ketoesters undergo photoreduction to yield pinacols.
C6H5COCOOC2H5 CH3 CH C2H5
+
hv
OH
C6H5 C
OH
COOC2H5
C
OH
C6H5 COOC2H5
Pinacols
α _ Ketoesters

lactone from ethyl aceto acetate
Ethyl acectoester yields solvent adduct Instead of pinacol.
Adduct is than converted into a Lactone. (β – Ketoester)
CH3COCH2COOC2H5 CH3OH hv CH3 C CH2CO O
C2H5
OH
CH2OH
Aceto Aceti c Ester
Ethyl Actoacetate
_ Ketoester
+ +
OH
CH3 C CH2CO O
C2H5
CH2OH
CH3
OH
C
CH2 O
CH2
C
O
Lactone
hv
β

Norrish Type I reaction
Photochemical excitation of ketones results into the hemolytic
fission of the α - carbon - carbon bond. This process is known as
α - cleavage or norrish type I reaction. n - * excited state of
acetone undergoes a carbon-carbon cleavage yielding a methyl
radical and an acetyl radical.
CH3 C CH3
O
hv S or T
CH3 CH3C
[ ]
O
CH3
C
CH3 +
O

*
n
At room temperature two acetyl radical combines to form
biacetyl.
O
CH3C
2
O
CH3C C CH3
O

At temperature above 1000
C; acetyl radicals
decarbonylate to
produce ethane and carbon monoxide.
O
CH3C CH3 CO
+ CH3
2 CH3 CH3
;
In case of unsymmetrical ketone, splitting takes place in a way
to generate the more stable of the two possible radicals.
CH3COCH( CH3) 2
CH3COCH2CH3
hv
CH3CO ( CH3) 2CH
CH3CH2
CH3CO
+
+

Norrish Type II reaction
Ketones having a γ - hydrogen atom undergo photochemical
reaction, which results in the formation of an olefin and the enol
of a smaller ketone. This process is known as Norrish type II
reaction.
CH
CH2
CH2
H
O
C
R
R'
R
C
CH2
CH2
CH
HO
R CH CH2 +
==
__
R
C
CH2
HO
CH3
C
O R'
R'
R R'
O
H
C
CH
CH2
CH2
α
β
γ
n 
*
hv
[ ]

Norrish Type I
Norrish Type II
]
[
+ C
O
CH3 CH3
C
O
O
C
+
CH2
CH3 CH2
CH2
CH3
CH2 CH2
+
+
CH3
CH2
CH2
CH2
CO
CO
CH3
CH3
CH3
CH3
CH3
+ H3C
OH
CH3
O
C
CH3
+
CH2
CH3
CH3
HO
H
C
CH2
CH2
CH2
CH2
CH2
CH2
C
O
CH3 CH3
CH2
CH2
O
C
CH2
Norrish type I and II reaction for 2-pentanone.

Photochemical reaction of cyclic ketones.
The photochemical reaction of saturated cyclic ketones
begin
with the n - * excitation of the carbonyl group followed by
α – fission. The biradical may then undergo hydrogen
transfer
from the α–carbon atom to the carbonyl group yielding a
ketene
or it may give rise to an unsaturated aldehyde by hydrogen
transfer to the carbonyl group.
O O
_ CO
hv

*
n
2,2|
-dimethyl cyclohexanone converted to 1,1|
-dimethyl cyclopentane .

cyclic ketones may give rise to an unsaturated aldehyde by
hydrogen transfer to the carbonyl group.
O O
hv

*
n
H
H
H
H
H
O
O
H H
O
H
O
CH
C
H
Keten
Aldehyde
_ di methyl
Cyclohexanone
α α
,
,

Paterno-Buchi Reaction
Photocycloaddition of carbonyl compounds with olefins upon
irradiation yields oxetanes, is known as paterno-buchi reaction.
n 
*
hv
R C
O
R'
R
+
C
C
R'
R'
R' O
R
R
R'
R'
Oxetanes
Photocycloaddition is performed by irradiation with the light of
wave length absorbed only by the carbonyl group, it involves the
triplet excited state of the carbonyl comp rather than that of olefin.

Mechanism of pateron-buchi reaction
O
C
CH2
CH3
( C6H5) 2 C
CH3
CH2
( CH3) 2C
+
3
[ ( C6H5) 2CO]
( C6H5) 2CO
1
[ ( C6H5) 2CO] 3
[ ( C6H5) 2CO]
I. S. C
.
hv
( i)
( ii)
Unsymmetrical Olefins:
Since the olefin is unsymmetrical, two products will be obtained.
The product in which the most stable biradical intermediate
forms, will be the major and other will be minor.

Mechanism of pateron-buchi reaction
hv I. S. C
. 3
[ ( C6H5) 2CO]
1
[ ( C6H5) 2CO]
( C6H5) 2CO
3
[ ( C6H5) 2CO] + ( CH3) 2C CH2
O C
CH2
CH3
( C6H5) 2 C
CH3
CH3
C
( C6H5) 2 CH3
CH2
C
O
CH3
C
CH3
CH2
C
O
O
C
CH2
CH3
C6H5
C
CH3
C6H5
C6H5
C6H5
more stable
l ess stable
Major ( 90%)
Mi nor( 10%)

(ii) Symmetrical Olefins:
Irradiation of benzophenone with either cis or trans-2-butene
yields the same mixture of both isomeric oxetanes.
( C6H5) 2CO
1
[ ( C6H5) 2CO] 3
[ ( C6H5) 2CO]
I. S. C
.
hv
3
[ ( C6H5) 2CO]
C6H5
CH3
C
C6H5
CH3
C
O C
C
O
C
CH3
C6H5
C
CH3
C6H5
C
O
C
CH3
C6H5
C
CH3
C6H5
+
+
CH3
CH
CH
CH3
( i)
( ii)
Cis - Trans
2- Butene

Limitation of paterno-buchi reaction
Paterno-buchi reaction fails when the triplet excitation energy of
ketone exceeds that of olefins. Under such condition oxetane
formation does not take place but energy transfer take place
from triplet excitation state of ketone (T1
) to the olefins (S0
).
There by olefin is excited to triplet state and is dimerize. e.g.
Irradiation of acetone with Norborene, yields dimmers of
norbornene rather than Oxetanes.

( i) CH3COCH3
1
[ CH3COCH3] 3
[ CH3COCH3]
I. S. C
.
hv
3
[ CH3COCH3] + CH3COCH3 +
( ii)
3
[ ]
]
3
[
( iii )
Di mer of Norborene

When a ketone (Benzophenone) having less triplet energy is used
will leads to paterno – buchi reaction to form Oxctanes of a
nonrborene.
( i)
hv I. S. C
. 3
[ ( C6H5) 2CO]
1
[ ( C6H5) 2CO]
( C6H5) 2CO
( ii )
3
[ ( C6H5) 2CO] +
O
C6H5
C6H5
hv

Paterno – buchi reaction also fails to occur with
conjugated dienes.
( C6H5) 2CO
hv
C4H9
C
C
C4H9
+ C6H5 C
C
C
O
O
C6H5
C
C6H5
3
]
[
T1
C4H9
C4H9
hv
C4H9
C4H9
O
C
C
C
C6H5
C6H5 C
C
C
O C4H9
C4H9

Photochemistry of α, β – unsaturated
ketone:
α, β – unsaturated ketone displays two absorption maxima at 220
nm ( - *) and at 310 nm(n - *) and irradiation may induce
either of these transition. The excited state in such ketone has
more  -* transition than n - * triplet even though there may be
an initial n - * excitation.
Two important reactions initiated by  - * states of the α, β –
unsaturated ketone
(i) Olefin addition across the double bond. and
(ii) photocyclodimerization.

(i) Olefin addition:
Photocycloaddition reaction involves, initial formation of a
carbon – carbon bond between the α–carbon of α, β–unsaturated
ketone and the olefin, followed by cyclization.
Cyclopentenone + Cyclopentene
O O
1
[ ] ]
3
[
O
O
3
[ ] +
n 
*
hv
( i )
( ii )
O O

*
n_
Cycl opentenone Cycl opentene
T1
]
[ _ 
*

 
*
_
[ ] T1

Cyclohexenenone + 1, 4, - dimethoxyethylene
O
+
CH2
CH3O
H
 
*
_
[ ] T1
OCH3
hv

*

O
H H
O H
OCH3 OCH3
OCH3 OCH3
 
*
_
[ ] T1
O
3
[ ] C
C
CH3
CH3
+

hv

*
O
CH3
CH3
Cyclopentenone + Acetylene

(ii) Photodimerization:
The photodimerization of α, β – unsaturated ketones occurs
readily in cyclic systems. Irradiation of cyclopentenone yields
head to head to and head to tail dimmers in almost equal amount.
However, head to head dimmers are preferred in polar solvent,
while head to tail dimmers are favoured in non – polar media.
O O O
O
O
H H H H
H H
H
H

*
hv

+
Cyclopentenone

The dimerization of cyclohexenene derivative occurs in a
similar
manners.
hv
+
O O O
O
O

Photochemistry of olefins:
The photochemistry of olefin involves only two types of electronic
excitations i.e. σ-* and -*. Promotion of an electron from σ-*
requires more energy (available from the light of wavelength
longer then 150 nm) and hence difficult to achieve under usual
experimental conditions. The -* excitation is experimentally
accessible as it requires the absorption of light of about 180 – 210
nm for non - conjugated olefins and of about 220 nm or more for
the conjugated olefins. Because of this reason most of the
photochemical studied involve only -* excitation.

Photoisomerization of cis and trans
Stilbene.
Direct irradiation of solution of either cis – or trans – stlbene yields
a mixture consisting of 60% cis – stilbene and 40% trans- stilbene.
C6H5CH CHC6H5
C6H5
C
C
H
H
C6H5
hv
C6H5
H
H
C
C
C6H5
+
Sti lbene
Ci s- Sti lbene Trans- Sti lbene

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