Dr.wael elhelece photochemistry 431chem

Photochemistry
Dr. Wael A. El-Helece

Electronic excitation of atoms and molecules.
Excited states of polyatomic molecules.
Kinetics of electronic excited state.
Electronic energy transition.
Chemical reactivity of excited electronic molecules.
Photo-electronic and photo-ionic spectra.
Diffraction of light in laboratory and outdoor
(environment).

Contents
Principles
Spectral regions
Applications
Experimental set-up
Excitation
Organic photochemistry
Inorganic and organometallic photochemistry
Carbon nanotubes
References

Introduction
Heat Electricity Electromagnetic
irradiation (light)
Energy

Photochemistry
Chemical reactions accompanied with light.
•
1. Action of light → chemical change (light induced reactions)
2. Chemical reaction → light emission (chemiluminescence)

Photochemistry
Study of chemical reactions that proceed with
the absorption of light by atoms or molecules.

Principles
Grotthuss–Draper law
light must be absorbed by a chemical substance in order for a
photochemical reaction to take place.
For each photon of light absorbed by a chemical system, no more than
one molecule is activated for a photochemical reaction, as defined by the
quantum yield.

Spectral regions
Ultraviolet: 100–400 nm
Visible Light: 400–700 nm
Near infrared: 700–2500 nm

Primary Processes
• One molecule is excited into an electronically excited state by absorption of
a photon, it can undergo a number of different primary processes.
•
• Photochemical processes are those in which the excited species
dissociates, isomerizes, rearranges, or react with another molecule.
•
• Photophysical processes include radiative transitions in which the excited
molecule emits light in the form of fluorescence or phosphorescence and
returns to the ground state, and intramolecular non-radiative transitions in
which some or all of the energy of the absorbed photon is ultimately
converted to heat.
•

What is Photochemistry about?
• Photochemistry is concerned with the changes in chemical and physical
behaviour of molecules following absorption of one (or more) photons.
• Primarily consider absorption of visible/UV although IR absorption may also
change chemical behaviour
*Mainly concerned with electronic excitation, usually accompanied by some
vibrational excitation (and rotational in gas phase ) excitation.

Chemistry of excited states
• Electronic excitation
change of molecular orbital occupancy
increased energy
change of bonding characteristics and possibly
geometry
change of charge distribution

possible changes of resultant electron spin, orbital
symmetry
Change of
Lifetime
Electron donating/accepting ability
Acid/base characteristics
Symmetry or energetic constraints on reaction

Excited states of formaldehyde
Resembles alkoxy
radical
No free radical
properties

Fig 2: Jablonski Diagrams

Significance of photochemical processes
• Atmospheric and astrophysical chemistry
• Photosynthesis
• Lasers
• Solar energy
• Semiconductor etching
• Biological damage – skin cancer etc
• Vision
• New chemistry
• Chemical Dynamics

Chemiluminescence:
P4 (g) + O2 (g)+H2O (g) P4 O10 + hυ green
Bioluminescence:
- Mushrooms
- insects
- fishes
Luminescence

Definitions and terms
Light: electromagnetic field vibration
spreading in quanta
(photons)
Photon: the smallest amount of light
carrying energy

Energy of photons (A. Einstein)
E = ch h=
h = Planck’s constant (6.6 · 10-34 Js)
c = speed of light (3 · 108 ms-1)
l = wavelength
n = frequency

Einstein’s Equivalency Principle
One particle of a chemical substance can absorb onlyone photon
from a light beam:
ΔE = hn
For one mole: ΔE = Nhn
N = Avogadro’s number (6.02 x1023)

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
1. Only light that is absorbed can produce photochemical
change (Grotthus, Draper)
2. A molecule absorbs a single quantum of light is becoming
excited (Stark, Einstein)

Mechanisms of Light Absorption
Excitation
X2
h *X2
A bonding electron is lifted to a higher energy level (higher orbital)

Interaction of Light and Materials
a) excess energy transferred to the surrounding.
X2* → X2 + M*
b) fluorescence or phosphorescence.
X2* → X2 + hυ
c) excess energy supplies the activation energy of the reaction.
X2* + Y → chemical reactionDr. Wael A. El-Helece

h
X2 X + X (photodissociation)
(energy of the photon supplies the dissociation heat)
Types of photochemical reactions
a) Photodissociation
b) Photosynthesis:
when a larger molecule is formed from simple ones.
c) Photosensitized reactions:
when an excited molecule supplies activation energy for the
reactants.

Photodissociation
Photolysis of hydrogen bromide
HBr
h
H + Br (photochemical reaction)
H + HBr H2 + Br
Br + Br Br2
(dark reactions)
Overall:
2HBr
h
H2
+
Br2

Note:
1 photon absorbed, 2 molecules of HBr dissociated:
QUANTUM YIELD =
2
1 = 2
number of molecules undergoing the process
number of quanta absorbed
=

Ozone formation in the atmosphere (at about 25 km altitude)
O2 O + O (λ ˂240 nm)
O2 +2O (+M*) 2O3 (+M*)
Note: M absorbs energy released in the reaction
M
Quantum Yield = 2/1 = 2
hυ

Ozone formed in the reaction above absorbs UV light as well:
O3 O2 + O (λ ˂340 nm)
O3 +O 2O2
Notes:
1. Ozone shield protects the Earth surface from high energy UV
radiation (of the Sun)
2. Air pollution (freons: fully halogenated hydrocarbons; nitrogen oxides
emitted by aeroplanes etc.) may accelerate the decomposition of ozone  ozone
hole
hυ

Photosynthesis
The photosynthesis of hydrogen chloride
Overall reaction:
Cl2 + H2 2HCl [no reaction in darkness]

Mechanism:
h
Cl2 < 500 nm
2Cl Photochem. initiation
Cl + H2 HCl + H Dark reactions
H + Cl2 HCl + Cl
Chain reaction
H + H + M H2 + M*
Cl2 + M*Cl + Cl + M
Recombination
reactions (chain
is terminated)
Note:
Quantum yield is about 106 (explosion)

Photosensitized reactions
Photosynthesis in plants
Overall reaction:
6CO2 + 6H2O C6H12O6+6O2
carbohydrate
h ; chlorophyll
several steps

Notes:
1.Chlorophyll acts as a catalyst absorbing and transferring the photon
energy for reduction of carbon dioxide to carbohydrate
2. This reaction maintains the life on the Earth:
sunlight carbohydrate
CO2; H2O
Fossile energy
(coal, oil, natural gas)
Food

Absorption
The Beer-Lambert Law
A beam of light (intensity I0) passes through a sample of
Length (l) with concentration (c).
I0lc
The intensity, I, of light transmitted through the sample
is given by the Beer-Lambert Law:
A = log10 I/I0 = e˂(v)cl

Photography
a)Photographic film: colloidal suspension of finely
powdered silver halogenide in gelatine
b) When exposed to light AgBr granuli become activated
according to the intensity of light
AgBr AgBr*h

Ago
AgBr*
developer
reduction
Unactivated granuli will be unaffected (but photosensitive!)
d) Fixation: Unaffected (photosensitive) AgBr should be removed:
AgBr + 2S2O3
2- [Ag(S2O3)2]3- + Br -
c) Development: Treating the exposed film with a mild reducing
agent the activated granuli will accelerate the reduction to metallic
silver (black).

Applications
*Photosynthesis.
*The formation of vitamin D.
*Photodegradation.
*Many polymerizations are started by photoinitiatiors.

Process of photosynthesis
6CO 22 6H O+
Sunlight
Chlorophyll
C H O O6
6 12 6 + 2
The carbohydrates so formed have been forming the
basis of life on earth.

So what are those funny symbols behind the O atoms and O2 molecules? Term
Symbols.
Spectroscopy: A Quick Qualitative Description
Term symbols show the energy state of atoms and molecules, as described by the
quantum numbers.
Atomic Quantum Numbers:
n – principal quantum number. Value: 1, 2, 3, ....
Tells which shell of an atom the e- resides. The farther from the nucleus the higher
the n.
l the azimuthal quantum number. Value: 0 to n-1.
Describes the orbital angular momentum of the shape of the orbital.
s – the spin quantum number. Value: ±½.
j – the total (spin plus azimuthal) quantum number.
Important for heavier atoms. Dr. Wael A. El-Helece

Spectroscopy: A Quick Qualitative Description, cont.
Energy states of Molecules: Molecular Quantum Numbers
L – the azimuthal quantum number. Value: 0 to n-1.
Orbital angular momentum
s – the spin quantum number. Value: ±½. Same as in atoms.
J – rotational quantum number. Value: 1, 2, 3, ....
Tells which shell of an atom the e- resides. The farther from the nucleus the higher the n.
n – vibrational quantum number. Value: 1, 2, 3, ....
K – vertical component of the total angular momentum. This QN only exists for polyatomic
molecules.
g/u – gerade/ungerade; symmetry terms. Reflection through the center of symmetry of
molecule.
+/- – plus/minus; symmetry terms. Reflection through the plane of symmetry of molecule.
Only for diatomics.

Sensitisation and Quenching
Certain reactions are known which are not
sensitive to light. These reactions can be made
sensitive by adding a small amount of foreign
material which can absorb light and stimulate
the reaction without itself taking part in the
reaction. Such an added material is known as
sensitiser and the process is sensitisation.
H
H
C
C COOH
COOH
Maleic acid
hv
Br
2
H
H
C
CHOOC
COOH
Fumaric acid

Quenching : - When a photochemical excited atom
has a chance to undergo collision with another atom
or a molecule before it fluoresces, the intensity of the
fluorescent radiation may be diminished or stopped.
This phenomenon is known as quenching.
Quenching is a radiationless process involving two
molecules.
A collision between a molecule in its excited state and
another chromophoric or reactive molecule is
quenching, the collision-induced, radiationless
relaxation of an excited state to the ground state.
The quenching process implies an interesting kinetic
competition, the treatment of which is referred to as a
Stern-Volmer analysis.

A* A
Lifetime of A* without Q = r = 1/k
1
11
Q A* A
k
+
kq
Lifetime of A* with Q = r2
][
1
][
1
1
1
2
Qk
r
Qkk
r
qq
Stern-Volmer quenching kinetics

Singlet and Triplet States and their Reactivity
It is essential to define some terminology with the
help of the following diagram
Fig 1: Spin orientation on the absorption of a light photon
Most molecules have an even number of electrons
and thus in the ground state, all the electrons are
spin paired.
The quantity 2S + 1, where S is the total electron
spin, is known as the spin multiplicity of a state.
hvhv
(a)(b) (c)
Antibonding
Orbital
Bonding
Orbital

1. Cis-Trans Isomerizations
When irradiated with uv-light olefins usually
undergo cis-trans isomerization.
The transformation can be carried out either by
direct irradiation of the olefins or by sensitized
irradiation.
It may either occur through a singlet or a
triplet excited species.
It has been reported that isomerization in the
triplet state has a lower barrier to rotation
around the carbon-carbon bond.

Photoisomerization of Stilbenest
Direct irradiation of solutions of either cis or
trans-stilbene gives rise to a constant mixture
having 93 % cis-stilbene and 7 % trans-stilbene.
Initial absorption of light by either of these
isomers has been found to be rapidly followed by
intersystem crossing to the corresponding triplet
states. Photoisomerization then takes place via
inter conversion or probably via a common triplet
intermediate.
C H CH6 5 CHC H6 5
hv
C
HH C65
C
H C65
H
+
C
HH C65
C
H C65
H
Cis-Stibene 93 % Trans-Stibene 7 %

Spectroscopy and Photochemistry Take Home Messages
1. The spectra of atoms and molecules are related to their ability to interact
with electromagnetic radiation, and to their shape and structure.
2. We use the observed spectra to determine the energy levels and geometry
of atoms and molecules.
3. Extraterrestrial radiation is absorbed by the atmosphere except in
window regions such as the visible and IR near 10 mm.
4. Transitions and reactions are influenced by selection rules, esp. spin
conservation.
5. The energy and lifetime set the natural line shape:
a. Rotations are slow, low energy, and very sharp.
b. Vibrations are intermediate.
c. Electronic transitions are very fast, high energy, and broad.

Spectroscopy and Photochemistry Take Home Messages, cont.
1. Oxygen: Schumann Runge Continuum <175 nm strong allowed.
Schumann Runge Bands < 200 nm
Herzberg Continuum < 242 nm forbidden weak.
2. Ozone: Hartley ~250 nm, allowed, strong.
Huggins < forbidden, weaker ~330 nm
Chappuis ~ 600 nm Forbidden, weak.
3. The production of OH and thus all of atmospheric chemistry depends strongly
on the wavelength dependent absorption of UV radiation.

Organic Photochemistry
Photochemical Process
[Gurdeep.R.Chatwal, Reaction Mechanism and Reagents in Organic Chemistry, Himalaya Publications, 2005, p 932]
Chapmann definition: - “It is the science which has
been arising from the application of photochemical
methods to organic chemistry and organic chemical
methods to photochemistry”.
Process of photosynthesis
The carbohydrates so formed have been forming the
basis of life on earth.
6CO 22 6H O+
Sunlight
Chlorophyll
C H O O6
6 12 6 + 2

Sensitisation and Quenching
Certain reactions are known which are not
sensitive to light. These reactions can be
made sensitive by adding a small amount of
foreign material which can absorb light and
stimulate the reaction without itself taking
part in the reaction. Such an added material
is known as sensitiser and the process is
sensitisation.
H
H
C
C COOH
COOH
Maleic acid
hv
Br
2
H
H
C
CHOOC
COOH
Fumaric acid

Quenching : - When a photochemical excited
atom has a chance to undergo collision with
another atom or a molecule before it
fluoresces, the intensity of the fluorescent
radiation may be diminished or stopped. This
phenomenon is known as quenching.
Quenching is a radiationless process involving
two molecules.
A collision between a molecule in its excited
state and another chromophoric or reactive
molecule is quenching, the collision-induced,
radiationless relaxation of an excited state to
the ground state.
The quenching process implies an interesting
kinetic competition, the treatment of which is
referred to as a Stern-Volmer analysis.

A* A
Lifetime of A* without Q = r = 1/k
1
11
Q A* A
k
+
kq
Lifetime of A* with Q = r2
][
1
][
1
1
1
2
Qk
r
Qkk
r
qq
Stern-Volmer quenching kinetics

Singlet and Triplet States and their Reactivity
It is essential to define some terminology with
the help of the following diagram
Fig 1: Spin orientation on the absorption of a light photon
Most molecules have an even number of
electrons and thus in the ground state, all the
electrons are spin paired.
The quantity 2S + 1, where S is the total electron
spin, is known as the spin multiplicity of a state.
hvhv
(a)(b) (c)
Antibonding
Orbital
Bonding
Orbital

1. Cis-Trans Isomerizations
When irradiated with uv-light olefins usually
undergo cis-trans isomerization.
The transformation can be carried out either
by direct irradiation of the olefins or by
sensitized irradiation.
It may either occur through a singlet or a
triplet excited species.
It has been reported that isomerization in
the triplet state has a lower barrier to
rotation around the carbon-carbon bond.

Photoisomerization of Stilbenes
Direct irradiation of solutions of either cis or trans-
stilbene gives rise to a constant mixture having 93
% cis-stilbene and 7 % trans-stilbene.
Initial absorption of light by either of these isomers
has been found to be rapidly followed by
states. Photoisomerization then takes place via inter
conversion or probably via a common triplet
intermediate.
C H CH6 5 CHC H6 5
hv
C
HH C65
C
H C65
H
+
C
HH C65
C
H C65
H

o When the spins are paired { } as shown in Fig
(a), the upward orientation of the electron spin is
cancelled by the downward orientation so that
S=0. This is illustrated below:
s1 = ½ ; s2 = – ½ so that S= s1+s2 = ½ – ½ = 0
o Hence, 2S + 1 = 1. Thus, the spin multiplicity of
the molecule is 1. We express it by saying that the
molecule is in the singlet ground state.
o When the absorption of a photon of a suitable
energy h , one of the paired electrons goes to a
higher energy level (excited state), the spin
orientation of the two singlet electrons may be
either parallel { } or antiparallel, { }, as shown in
Fig (b) and (c) respectively.

 If the spins are parallel, as shown in Fig
(b), then, S= s1+s2 = ½ + ½ =1 so that 2S+1=3.
 Thus, the spin multiplicity of the molecule is 3. This
is expressed by saying that the molecule is in the
triplet excited state.
 If however, the spins are antiparallel, as shown in
Fig (c), then, S= s1+s2 = ½ – ½ = 0 so that
2S+1=1. Thus, the spin multiplicity of the molecule
is 1. This is expressed by saying that the molecule
is in the singlet excited state.
 Since the electron can jump to any of the higher
electronic states depending upon the energy of the
photon absorbed, we get a series of the singlet
excited states, Sn where n=1, 2, 3, 4 ……and a
series of triplet excited states, Tn where
n=1, 2, 3, 4 ……

Thus, S1, S2, S3………. are known as the first
singlet excited state, second singlet excited
state, third singlet excite state……..etc.
Similarly, T1, T2, T3……….. are called the first
triplet excited state, second triplet excited
state, third triplet excited state….etc.
It has been shown quantum mechanically that
a singlet excited state has higher energy than
the corresponding triplet excited state.
Accordingly, the energy sequence is as shown
below.
and so on332211 TSTSTS EE;EE;EE

On absorption of light photon, the electron of the
absorbing molecule may jump form S0 to S1,S2 or
S3 singlet excited state depending upon the
energy of the photon absorbed as shown in
Jablonski diagram [Fig: 2].
For each singlet excited state (S1, S2, S3……….
), there is a corresponding triplet excited state
(T1, T2, T3……….. )’
The molecule, whether in singlet or triplet excited
state, is said to be activated. Thus;
where A0 is the molecule in the ground state and
A* is the molecule in the excited state.
The activated molecule returns to the ground
state by dissipating its energy through the non-
radiative and radiative transition process.
A*+A0 hv

Photoreactions of Carbonyl Compounds;
Enes, Dienes & Arens
[Gurdeep.R.Chatwal, Reaction Mechanism and Reagents in Organic Chemistry, Himalaya Publications, 2005, p 959-961]
Only two types of electronic excitations is
possible in the photochemistry of enes; to *
Promotion of an electron from to * needs
and to *.more energy (available only from
the light of wavelength lower than 150 nm).
Therefore, it is difficult to take place under
usual experimental conditions.
to * excitation has been experimentally
accessible because it needs the absorption of
the light of about 180-210nm for
nonconjugated olefins and of about 220 nm or
more for conjugated olefins.

The initial excitation ( to *) usually takes place
with no change in multiplicity and so a singlet
excited state is formed.
Unlike n to * transitions of ketones, this transition
has been symmetry-allowed and thus results in a
strong absorption band.
The singlet excited state of olefins possesses less
tendency to intersystem crossing and they
themselves could initiate many photochemical
reactions.
However, the T1 states of olefins have been readily
formed by intermolecular energy transfer from
triplet donor to an olefin molecule.
The photochemistry of singlet excited state of an
olefin is appreciably different from that of its triplet
state.

1.Cis-Trans Isomerization of Stilbene
Olefins usually undergo cis-trans isomerizations
when irradiated with uv-light.
The transformation can be carried out either by
direct irradiation of the olefins or sensitized
irradiation.
It may either occur through a singlet or a triplet
excited species.
It has been reported that isomerization in the
triplet state has a lower barrier to rotation
around the carbon-carbon bond because simple
olefins absorb light at about 200 nm.

• The photoisomerization of the stilbenes has been
probably the direct irradiation of solutions of
either cis or trans-stilbene gives rise to a
constant mixture having 93 % cis-stilbene and 7
% trabs-stilbene.
• Initial absorption of light by either of these
isomers has been found to be rapidly followed by
state.
• Photoisomerization then takes place via inter-
conversion or probably via a common triplet
intermediate.
C H CH6 5 CHC H6 5
hv
C
HH C65
C
H C65
H
+
C
HH C65
C
H C65
H

2.Dimerization Reaction
o In this process there occurs the generation of
an excited triplet molecule which subsequently
reacts with a ground state molecule.
o A well-known example involves the acetone-
sensitized photodimerization of norbornene.
o There may occur an intramolecular reaction
between two properly situated double bonds in
a molecule forming an isomeric substance.
hv
Acetone

3. Addition reaction of cyclic olefins
Cyclic olefins are also known to undergo
addition reactions, on irradiation in methanol.
The reaction of (I) with methanol has been
reported to be sensitized by xylene.
+ CH OH3
hv
Xylene
H C OCH33
CH O CH3 3
(I)

Photochemistry of butadiene
Butadiene is known to exist in solution as a
mixture of S-trans (95 %) and S-cis (5 %)
conformers.
In the irradiation of butadiene, an electron gets
promoted from 2 to 3 ( to * transition)
which gives rise to the increased bonding
between C2 and C3 at the expense of C1------C2
and C3------C4.
Hence, conformational character of butadiene
gets retained in the excited states.
95 % trans 5 % cis

Direct irradiation of butadiene gives rise to
cyclobutene (I) and bicyclo butane (II).
The formation of these products directly from the S1
state of the butadiene.
The conformational characters of butadiene get
retained in the S1 state, it is quite reasonable to
speculate the S-cis butadiene has been the
precursor of cyclobutene whereas the excited state
resembling S-trans butadiene yields bucyclobutane.
.
.hv
hv
..
hv
and +
(I) (II)

Norrish reactions of acyclic ketones
Photochemical excitation of ketones usually
causes the homolytic fission of the -carbon-
carbon bonds.
This process is called -cleavage or Norrish
type I reaction.
Acetone which gets photolyzed in the vapour
phase as well as in the liquid phase.
Abaorption of light gives rise to the formation
of an n to * excited state of acetone which
undergoes a carbon-carbon cleavage to form
a methyl radical and an acetyl radical.

At room temperature, two acetyl radicals
undergo combination to form biacetyl.
At temperature above 100oC, acetyl radicals
get decarbonylated with the ultimate formation
of ethane and carbon monoxide.
CH CCH33
O
hv
CH CCH33
O
CH3
. + CH C
O
3
.
O
3
.CH C2 CH C C CH3
OO
3
O
3
.CH C
2 3 3
CH
.
3
+ CO
CH
.
3
CHCH

The Paterno-Buchi Reaction
Carbonyl compounds on irradiation in the presence
of olefins yield oxetanes. This photocycloaddition is
generally known as the Paterno-Buchi Reaction.
The addition is carried out by irradiation with the
light of wavelength absorbed only by the carbonyl
group.
The light energy needed for the n to * transition is
able to initiate the reaction in simple cabonly
compounds.
O
+C
RR
C
C
R
R
R
R
hv
R
R
O
R
R
R
R

Barton reaction
The Barton Reaction involves the photolysis of a
nitrite to form a δ-nitroso alcohol.
The mechanism is believed to involve a homolytic
RO–NO cleavage, followed by δ-hydrogen abstraction
and free radical recombination.

Photo-Fries rearrangement
Photo-Fries rearrangement involves a radical
reaction mechanism.
This reaction is also possible with deactivating
substituents on the aromatic group.
Because the yields are low this procedure is not
used in commercial production.
However, photo-Fries rearrangement may occur
naturally particular to UV light at a wavelength of
about 310 nm.

Di- methane rearrangement
The Di- methane rearrangement is a photochemical
reaction of a molecular entity comprising two -
systems, separated by a saturated carbon atom (a
1,4-diene or an allyl-substituted aromatic analog), to
form an ene- (or aryl-) substituted cyclopropane.
The rearrangement reaction formally amounts to a
1,2 shift of one ene group (in the diene) or the aryl
group (in the allyl-aromatic analog) and bond
formation between the lateral carbons of the non-
migrating moiety

Photochemical conversion of Ergosterol to Vitamin D2
Ergosterol is a biological precursor (a provitamin) to
vitamin D2.
It is turned into viosterol by ultraviolet light, and is
then converted into ergocalciferol, a form of vitamin
D also known as D2 .
 For this reason, when yeast (such as brewer's yeast)
and fungi (such as mushrooms), are exposed to
ultraviolet light, significant amounts of vitamin D2 are
produced.
Ergosta-5,7,22-trien-3β-ol

Singlet Oxygen Generation and Reaction
• The lowest excited singlet state of O2 lies by
only 94 kJ mol-1 above the triplet ground state.
This 1Dg state is commonly populated by
electronic energy transfer from photoexcited
sensitizers.
• Due to its excitation energy of 94 kJ mol-1
singlet oxygen is chemically extraordinary
reactive.
• The chemistry of singlet oxygen is different
from that of ground state oxygen. For
example, singlet oxygen can participate in
Diels-Alder [4+2] and [2+2] cycloaddition
reactions, ene reactions

citronellolAn example is an oxygenation of
Singlet_Oxygenation_Citronellol

Applications of photoreactions in synthesis
Many important processes involve photochemistry. The premier
example is photosynthesis, in which most plants use solar energy to
convert carbon dioxide and water into glucose, disposing of oxygen
as a side-product.
Humans rely on photochemistry for the formation of vitamin D. In
fireflies, an enzyme in the abdomen catalyzes a reaction that results
in bioluminescence.
Photochemistry can also be highly destructive. Medicine bottles are
often made with darkened glass to prevent the drugs from
photodegradation.
A pervasive reaction is the generation of singlet oxygen by
photosensitized reactions of triplet oxygen. Typical photosensitizers
include tetraphenylporphyrin and methylene blue. The resulting
singlet oxygen is an aggressive oxidant, capable of converting C-H
bonds into C-OH groups.
In photodynamic therapy, light is used to destroy tumors by the
action of singlet oxygen.
Many polymerizations are started by photoinitiatiors, which
decompose upon absorbing light to produce the free radicals for
Radical polymerization.

• Photochemical reactions are not only very useful but
also can be a serious nuisance, as in the
photodegradation of many materials, e.g. polyvinyl
chloride and Fp.
• A large-scale application of photochemistry is
photoresist technology, used in the production of
microelectronic components.
• Vision is initiated by a photochemical reaction of
rhodopsin

Dr.wael elhelece photochemistry 431chem

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Dr.wael elhelece photochemistry 431chem