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PHOTOCHEMISTRY
BY
Dr. Bhawna
INTRODUCTION
• Photochemistry is defined as that branch of
chemistry which deals with the study of interaction of
radiation with matter resulting into a physical change or
into a chemical reaction (called photochemical reaction)
whose rates and mechanisms are studied after initiation by
the radiant energy. Though the term radiation includes all
types of electromagnetic waves from very low frequency
microwaves to medium frequency infrared, visible and
ultraviolet radiation to high frequency X-rays and γ-rays, the
radiation of photochemical importance are those which lie in the
visible and ultraviolet region from 8000 - 2000 Å (800 - 200
nm).
• Thus the two main processes studied under photochemistry are
:
• (i) Photophysical processes
• (ii) Photochemical processes or photochemical reactions.
Photophysical processes
• Photophysical processes are those processes
which take place in the presence of light but do
not result into any chemical reaction. These
processes arise on account of the absorption of
light by the substances followed by the emission of
the absorbed light. If the absorbed light is emitted
instanteniously, the process is called
fluorescence. If the absorbed light is emitted after
some time lag, the process is called
phosphorescence. Further, if the energy of the
light absorbed is sufficiently high the electrons may
not only jump to the outer levels but may leave the
atoms completely ; the process is called
photoelectric effect. Thus photophysical
processes include phenomena like fluorescence ,
phosphresence and photoelectric effect.
• Photochemical reactions are those reactions
which take place by absorption of light by the
reacting substances. These reactions are generally
brought about by the absorption of light radiations of the
visible and ultraviolet region which lie between 8000 2000 Å (800-200 nm) as already mentioned above. In
these cases, the light energy absorbed is stored within
the substance and then used for bringing about the
reaction. A large number of different types of reactions
can be brought about by exposure to suitable light e.g.
synthesis, decomposition, oxidation, reduction,
polymerisation, isomeric change etc.
• Two well known examples of photochemical
reactions are given below :
• (i) Combination of hydrogen and chlorine to
form hydrogen chloride
•
H2(g) + cl2(g)
Light→ 2HCl(g)
•
(ii) Photosynthesis of carbohydrates in
plants taking place in presence of chlorophyll.
the green colouring matter present in the leaves
•
6C02 + 6H20 → C6H1206 + 602
•
From air
Glucose
DIFFERENCE BETWEEN PHOTOCHEMICAL
REACTIONS AND THERMOCHEMICAL REACTIONS
Thermochemical Reactions
Photochemical reactions
(i)
These reactions involve
absorption of heat.
These reactions involve absorption of
light.
(ii)
They can take place even in the
dark.
The presence of light is the primary
requisite for the
reaction to take place.
(iii)
Temperature has significant
effect on the rate
Temperature has very little effect on the
rate of a
of a thermochemical reaction.
photochemical reaction. Instead, the
intensity of light has
a marked effect on the rate of a
photochemcal reaction
iv)
The free energy change (ΔG) of The free energy change (Δ G) of a
a thermochemical
photochemical reaction
reaction is always negative.
may not be negative. A few examples of
photochemical
reactions for which ΔG is positive and still
they are spontaneous
(i)Synthesis of carbohydrates in plants.
(ii)Ozonization of oxygen.
(iii)Decomposition of HCl into H2 and Cl2*
iv)
In these reactions, activation
energy arises
In these reactions, activation energy is
acquired by absorption
from the intermolecular
collisions or
of photons of suitable energy
it is supplied in the form of
heat.
(vi)
When a reaction mixture is exposed
to heat
When a reaction mixture is exposed to
light radiation, the
radiation, all the reactant molecules
absorb
molecules of one of the reactants may
get excited
these radiations and get excited
almost to
independent of the presence of other
species
the same extent i.e. there is no
selectivity
(eg .only Br2 molecules in a mixture of
H2 and Br2)
LAWS GOVERNING
ABSORPTION OF LIGHT
• When a monochromatic light (light consisting of
a single wavelength) is passed through a
medium some light is absorbed by the medium
and hence the intensity of the incident light
decreases. The decrease of intensity on
passing through a pure homogeneous
medium (i.e. a pure liquid) is governed by
Lambert's law and the decrease of intensity on
passing through a solution is governed by
Beer's law.
Lambert's Law .
• This law put forward by Lambert (in 1760)
states as under
When a monochromatic light is passed
through a pure homogeneous medium, the
decrease in the intensity of light with
thickness of the absorbing medium at any
point X is proportional to the intensity of the
incident light i.e. the intensity of light at the
point X (i.e. just before entering into the dx).
Mathematically,
• -dl / dx α I or -dl / dx =kI
•
•
•
•
•
•
where dl is the small decrease in intensity of the light
on passing through a small thickness dx, I is the
intensity of the incident light just before entering the
thickness dx and the constant of proportionality k is
called the absorption coefficient depending upon the
nature of the absorbing medium.
The intensity I at any point X (at a distance x from the
start of the medium), can be found in terms if the
original intensity I0 as follows :
Equation (1) can be written as
dI/ I= -k dx
...(2
When x = 0,I = I0
x = x, I =I
Integrating equation (2) between the limits x = 0 to x and I= I0 to I, we
get
This equation expresses how the original intensity I0 is
reduced to intensity I after passing through a thickness x of
the medium*. The difference between I0 and I obviously
gives the intensity of the light absorbed.
• Thus
• Iabs =I0 - I = IO-I0 e –kx
• Equation (3) can be written as
•
2.303 log I/I0 = -kx
•
log I/I0 = - kx/2.303
•
log I/I0 = - k ‘x
•
I/I0 = 10 - k ‘x
(6)
•
I = I0 x 10 - k ‘x
• Where k’= k/2.303 is called extinction coefficient of
the substance ( i.e. of the absorbing medium). Now it
is this quantity which is called absorption coefficient
or absorptivity of the substance.
• The physical significance of the extinction
coefficient or absorptivity follows more clearly from
equation (6) which can be rewritten as
•
K’ = -1/x log I/I0 = 1/x log I0 /I
•
If
log I0 /I = 1 , K’= 1/x
•
But
log I0 /I = 1 means I0 /I = 10
•
i.e.
I = IO/10
•
• Hence extinction coefficient or absorptivity may be
defined as follows :
• The extinction coefficient or absorptivity is the
reciprocal of that layer thickness (expressed in
cm) at which the intensity of the light falls to onetenth of its original value.
Beer's Law
• . This is an extension of Lambert' law, put forward by
Beer (in 1852). It is applicable to solutions and states
as under :
• When a monochromic light is passed through a
solution, the decrease in the intensity of light with
the thickness of the solution is directly
proportional not only to the intensity of the
incident light but also to the concentration 'c' of
the solution. Thus for a solution, mathematically, we
have
• -dl/dx α I x c
• -dl/dx = εIc (ε is pronounced as epsilon)
(8)
• where ε is a constant of proportionality and is called
molar absorption coefficient. Its value depends
upon the nature of the absorbing solute and the
wavelength of the light used.
• Equation (8) can be rewritten as
•
dI/I = - εc dx
• As before, integrating this equation between the
limits x = 0 to x and I= I0 to I , we get
•
ln I/IO = - εcx
• Or
I/I0 =e - εcx
• Or
I =I0 e - εcx
• This equation shows that the intensity of a
monochromatic light decreases
exponentially from I0 to I with increase in
thickness x and concentration c.
• The intensity of the light absorbed by the solution
when it passes through a length x of the solution
will be given by
• Iabs = I0 - I = I0 - I0 e- εcx = I0(I- e- εcx) ..(12)
•
•
•
•
•
•
2.303 log I/ I0 = - εcx
or
logI/I0=- εcx/2.303
or logI/I0=- έcx
_(13)
or
I/I0= 10 - έcx
or
I = I0 x 10 - έcx
where έ = ε/ 2.303 was earlier called molar
extinction coefficient of the absorbing solution.
Now it is this quantity which is called molar
absorption coefficient or molar absorptivity of the
absorbing solution.
• The physical significance of molar extinction
coefficient or molar absorptivity, follows from
equation (13), which can be written as
• έ = -1/cx log I/I0 = 1/cx log I0/I
• If c=1 M and log I0/I = 1 then έ = 1/x
• As before, log I0/I = 1 means I0/I = 10 i.e. I = I0/10
• Hence molar extinction coefficient or molar
absorptivity may be defined as the reciprocal of
that thickness of the solution layer of 1 molar
concentration which reduces the intensity of the
light passing through it to one-tenth of its original
value.
• It may be pointed out that gases follow Beer's law and
not Lambert's law.
• Units of molar absorptivity or absorption
coefficient
• έ = - 1 /cx log I/I0
• log I/I0 is a dimensionless quantity.
• In SI units, x is in metres (m) and c is in
mol m -3. Hence, we have
• έ=(m3 mol -1)(m -1)=m 2 mol -1
LAWS GOVERNING PHOTOCHEMICAL
REACTIONS
• There are two important laws of photochemistry
as explained below :
• 1. Grotthus-Draper Law. This law is also
called First law of photochemistry. It was put
forward by Grotthus on theoretical grounds (in
1818) and confirmed by Draper by experiment
(in 1839). It states as under:
• When light falls on a body, a part of it is
reflected, a part of it is transmitted and the
rest of it is absorbed. It is only the absorbed
light which is effective in bringing about a
• The above law, however, does not imply that the
absorbed light must always result into chemical
reaction. The absorbed light may simply bring about
phenomena such as fluorescence, phosphorescence
etc., Similarly, the absorbed light energy may be
simply converted into thermal energy e.g. in case of
potassium permanganate solution, the light energy is
absorbed strongly but no chemical effect is produced.
Further in some cases, it is observed that light energy
may not be absorbed by the reacting substance
directly but may be absorbed by some other
substance present alongwith the reacting substance.
The energy thus absorbed is then passed on to the
reacting substance which then starts reacting. This
process is called 'photo-sensitization' . A well known
example is that of photosynthesis of carbohydrates in
plants where chlorophyll acts as a photosensitizer.
• 2. Stark-Einstein's Law of Photochemical Equivalence.
• This law is also called second law of photochemistry. It was
put forward by Einstein (1905) and Stark (1908). It states at
under:
• Every atom or molecule that takes part in a photochemical
reaction absorbs one quantum of the radiation to which the
substance is exposed.
• If v is the frequency of the absorbed radiation, then the energy
absorbed by each reacting atom or molecule is one quantum
i.e. hv where h is Planck's constant. The energy absorbed bv
one mole of the reacting molecules will then be given by
•
E = Nhv
...(1)
• where N is Avogadro's number (i.e. the number of molecules
present in one mole of the substance)
• Putting v =c/λ We can write
E = Nh c/λ
(2)
• where c is the velocity of light and λ is the wavelength of the
absorbed radiation
• The energy possessed by one mole of photons (or
the energy absorbed by one mole of the reacting
molecules) is called one einstein.
• In CGS units
• E = 2.86 x 105 / λ (Å) kcal per mole
• In SI units
• E= 11.97 x 10 -5 / λ(m) kJ/mol
• Thus the energy per Einstein is inversely proportional
to the wavelength of the radiation. Shorter the
wavelength of a radiation , greater is the energy per
Einstein. For example the order of wavelength of
different radiations is
•
X rays < Ultraviolet < Visible < Infrared
• Hence the energies per Einstein of these radiations
are in the order
•
X rays > Ultraviolet >Visible > Infrared
INTERPRETATION OF EINSTEIN'S LAW -'QUANTUM EFFICIENCY'
• If Einstein's law is correct, every reacting molecule will absorb
one quantum of radiation. Hence the number of reacting
molecules should be equal to the number of quanta absorbed.
However, it is found that in a number of cases, a small amount
of the light absorbed can bring about a large amount of
reaction, whereas in some other cases, large amount of the
light absorbed can bring about only a small amount of reaction.
This was explained on the basis that all the molecules present
do not absorb the radiation, only a few molecules absorb the
radiation. These molecules are said to have been 'activated'
These activated molecules then react with the other molecules
and thus bring about a large amount of chemical reaction. On
the other hand, in certain cases, the activated molecules get
deactivated and no reaction takes place. The first stage
(involving the absorption of light) is called the primary
process. The reactions involved in the second stage are called
secondary processes.
• To relate the number of quanta absorbed with the number of
reacting molecules, a term called quantum efficiency or
quantum yield (φ) has been introduced. It is expressed as
• φ = Number of molecules reacting in a given time
•
Number of quanta of light absorbed in the same time
•
= Number of moles reacting in a given time
•
Number of einsteins of light absorbed in the same time .
•
Hence quantum efficiency may be defined as the number
of moles reacting per einstein. light absorbed.
• If Stark-Einstein's law is strictly obeyed in the form already
stated, φ should be equal to unity. However, studies have
shown that the law is applicable only to the primary processes.
Hence it is sometimes preferred to state it as follows :
• Each molecule that gets activated to initiate the reaction
absorbs one quantum of light.
EXPERIMENTAL DETERMINATION OF QUANTUM
YIELD OF A PHOTOCHEMICAL REACTION
• Theory of the process. For experimental purpose,
the expression for the quantum yield may be written
as
• φ = Number of moles reacting per sec
•
Number of quanta absorbed per sec.
•
= Rate of chemical reaction
•
Quanta absorbed per sec
• Thus to determine the quantum yield of any
photochemical reaction, we have to measure (i) the
rate of chemical reaction in terms of molecules per
sec.
• (ii) the quanta absorbed per sec.
• Apparatus. The apparatus used for measuring
the above two quantities (and hence for the
determination of the quantum yield) It consists
of the following parts from left to right:
• (i) A source of radiation emitting radiation of
suitable intensity in the desired special range.
The commonly used sources are filament
lamps, carbon metal arcs and various gas
discharge Tubes.
• (ii) Lens of suitable focal length.
• (iii) Monochromator or filter which cuts off all
radiations except the radiations of the desired
wavelength.
• (iv) Slit to get a fine beam.
• (v) A cell placed in thermostat and containing
the reaction mixture. The cell is made of glass
or quartz and has optically plane windows for
the entrance and exit of light. Glass is used only
if the wavelength of the light used lies in the
visible range. For radiations with wavelength
below 3500 Å, the cell completely made of
quartz is used.
• (vi) Recorder. It is usually a 'thermopile' or an
actinometer which is used to measure intensity
of the radiation. The principle of a thermopile
and that of an actinometer are briefly explained
as follows :
• A thermopile is a set of thermocouples. Rods of two
different metals (e.g. Ag and Bi) are joined alternately
as shown in Fig. (If only two rods are connected, it
constitutes a thermocouple). One set of junctions is
soldered to metal strip blackened with platinum or
lamp black and the other set of junctions is protected
completely from radiation by placing the system in a
box as shown in Fig.. The radiations falling on the
blackened metal strip are almost completely absorbed
by it. As a result, this set of junctions becomes hot.
The temperature difference of the hot end and the cold
end produces a current which can be measured by
connecting a milliammeter to the thermopile. By
calibrating the thermopile with radiations of known
intensity, the intensity of the desired radiations can be
measured.
METAL STRIP BLAKENED
WITH PT. BLACK
A THERMOPILE
• A chemical actinometer is a device in which gas
mixture or solutions sensitive to light are used The
working of this device is based upon the fact that a
definite amount of the radiation absorbed brings about
a definite amount of chemical reaction. The most
commonly employed is the uranyl oxalate actinometer
which consists of 0.05 molar oxalic acid and 0.01
molar uranyl sulphate (U02S04) solution in water.
When exposed to light, the reaction takes place
converting oxalic acid into CO2 and H2O :
• Uranyl sulphate acts as photosensitizer (i.e. absorbs
the light and then passes it on to oxalic acid). The
extent of reaction is measured by titrating the oxalic
acid solution with potassium permanganate solution.
The actinometer is first calibrated against a
thermopile. This actinometer can be used for
measuring only the intensity of the radiation lying in
the range 2000 - 5000 Å.
Procedure
• (i)Measurement of the intensity of light absorbed.
First, the empty cell or the cell filled with solvent alone
(in case of solutions), is placed in the thermostat. The
monochromatic radiation is allowed to pass through
the cell for a definite time. The reading is taken on the
recorder. From this we get the energy of the incident
radiation. Now the reactants are taken in the cell and
the radiations are allowed to pass through it for
definite time and the reading is taken again. The
difference between the two readings gives the
total energy absorbed by the reactants in a given
time. From this the intensity of the radiation absorbed
Iabs is calculated as follows :
• Iabs = Total energy absorbed
•
Volume of the reaction mixture x Time in sec
• (ii) Measurement of the rate of reaction. The
rate of the reaction can be studied as usual. For
this purpose, either the change in some
physical property is followed or samples of the
reaction mixture are removed periodically from
the cell and analysed.
• Knowing the rate of reaction and the intensity of
light absorbed, the quantum yield of the
reaction can be calculated.
PHOTOCHEMICAL REACTIONS
• A photochemical reaction takes place in two
steps
• 1. Primary process : This involves activation of
some molecules by absorption of light.
•
AB + hv → AB*
• Or AB + hv → A + B*
• 2. Secondary process : This involves reaction
of activated molecules of primary process with
other molecules resulting into activation or
deactivation
•
• On the basis of above processes
photochemical reactions are of three types
• 1. Quantum yield is a small integer.
•
H2 + Br2 → 2 HBr φ=1
• 2. Quantum yield is very large.
•
H2 + Cl2 → 2HCl
φ=10 6
• 3. Quantum yield is very small.
•
NH3 + hv → N2 + H2 φ=0.25
Photosynthesis of HBr i.e. Photochemical combination
of Hydrogen and Bromine to form HBr.
• The reaction may be represented as
• H2 + Br2 → 2 HBr
• The quantum efficiency of this reaction is very low i.e.
about 0.01 at ordinary temperatures. This is explained
by proposing the following mechanism for the above
reaction :
• (a)Primary process ;
•
Br2 + hv → 2 Br
• (b)Secondary Process :
•
(i) Br + H2 endo→ HBr + H
•
(ii) H + Br2 → HBr + Br
•
(iii)H + HBr → H2 + Br
•
(iv) Br + Br → Br2
Photosynthesis of HCl from H2 and Cl2
• This is an example of a reaction whose quantum yeild is very
high i.e. 10 4 to 10 6. This reaction may be represented as
• H2 + Cl2 → 2HCl
• The high quantum yield of this reaction is explained by the
chain mechanism (proposed by Nernst in 1918). The different
steps involved are as follows :
• (a) Primary process :
• Cl2 + hv → 2Cl Chain initiating step
• i.e. a chlorine molecule absorbs one quantum of light and
dissociates to give CI atoms.
• (b) Secondary process :
• (i) CI + H2 exo--→ HCl + H
|
Chain propagating steps
• (ii) H + Cl2 → HCl+ CI
I
• (iii) CI + CI →
Cl2 }
Chain terminating step
• It is interesting to note that the mechanism of
the above reaction is similar to that of the
photosynthesis of HBr, yet the quantum yield of
this reaction is very high whereas that of the
photosnthesis of HBr is very low. The difference
is explained on the basis of the reaction (i) of
the secondary processes. In the present
reaction, the reaction (i) of the secondary
processes which immediately follows the
primary process is exothermic and therefore,
takes place very easily and consequently the
chain reaction is set up very easily. In the
photosynthesis of HBr, the reaction (i) of the
secondary process is endothermic and thus
has very little tendency to take place .
• The quantum yield of the above reaction decreases if
the reaction vessel contains small traces of oxygen.
This is explained on the basis that oxygen leads to the
chain termination by the following reactions
• H + 02 → H02
• CI + 02 + HCl → H02 + Cl2
• The chain mechanism for the photosynthesis of HCl,
as discussed above, is further supported by the
following facts :
• (i) The reaction can be brought-about without
exposing the reaction mixture (H2 + Cl2) to light but by
simply adding some CI or H atoms into the reaction
mixture.
• (ii) The quantum yield of the reaction decreases
considerably if the reaction is carried out in capillary
tubes. This confirms that the chains are terminated on
the walls of the reaction vessel.
Photolysis of Ammonia.
• This is an example of a photochemical reaction whose
quantum yield is very low i.e. about 0.25 at ordinary
temperatures and pressures. The probable
mechanism is supposed to be as follows :
• (a) Primary process:
• NH3 + hv → NH2 + H
• The presence of H-atoms has been proved by suitable
methods.
• (b) Secondary processes. The ultimate products of
the photolysis of amonia have been found to be
nitrogen, hydrogen and hydrazine. Hence the
following probable secondary reactions have been
proposed ;
• (i) NH2 + NH2 → N2+ 2H2
•
•
•
•
(ii) NH2 + NH2 → N2H4
(iii) H + H → H2
(iv) NH2 + H → NH3
Since the quantum yield of the reaction is very
low, the reaction (i) to (iii) take place only to a
very small extent. The important secondary
reaction is the reaction (iv) only which involves
the reversal of the primary process.
PHOTO- PHYSICAL PROCESSES
• Fluorescence. There are certain substances which
when exposed to light or certain other radiations
absorb the energy and then immediately or
instantaneously start re-emitting the energy. Such
substances are called fluorescent substances and the
phenomenon is called fluorescence. Obviously, the
absorption of the energy results into the excitation of
the electrons (present in the atoms or the molecules of
the substance) followed immediately by the jumping
back of the excited electrons to the lower levels .As a
result, the absorbed energy is emitted back..
• The word 'instantaneous' used above implies that the
time between the absorption of this energy and the reemission of the absorbed energy is not more 10 -8 sec.
Thus fluorescence starts as soon as the substance is
exposed to light and the fluorescence stops as soon
as the light is cut off.
• A few examples of the substances showing the
phenomenon of fluorescence are :
• (i) fluorite, CaF2 (from which the name of the
phenomenon is derived)
• ( ii) certain organic dyes such as eosin,
fluorescein etc
• (iii) vapour of sodium, mercury, iodine etc.
• (iv) certain inorganic compounds such as uranyl
sulphate,
•
It is interesting to observe that in the phenomenon
of fluorescence, the wavelength of she emitted light is
usually greater than that of the absorbed light. This is
explained on the basis that the energy absorbed
raises the electron to a sufficiently higher level but the
return of the excited electron to the original level takes
place in steps, through the intermediate levels.
• The energy thus produced in every jump is
smaller and hence the wavelength of the light
emitted is large (as E = hv = hc/λ i.e. E α 1/λ)
than that of the absorbed light.
• In case of mercury vapour, the emitted light has
the same wavelength as that of the absorbed
light . This phenomenon is called resonance
fluorescence. In certain cases, light of shorter
wavelength (i.e. of higher energy) is emitted.
This is explained by suggesting that the emitted
light contains the absorbed energy as well as
some energy present in the system before
absorption.
• Phosphorescence. In case of fluorescent
materials, the florescence stops as soon as the
external light is cut off. However, there are
certain substances which continue to glow for
some time even after the external light is cut off.
Such substances are called phosphors or
phosphoresce substances and the
phenomenon is called phosphorescence.
Thus in a way, phosphorescence is a slow
fluorescence. It is found mostly in solids, as
might be expected because in solid, the
molecules have least freedom of motion.
The excited electrons thus keep on jumping
back slowly for quite some time.
• The best examples of the substances exhibiting
phosphorescence include zinc sulphide and
sulphide of the alkaline earth metals. Usually
a trace of heavy metal is added (in the form of
its sulphide) which intensifies the light emitted
by the phosphorescent substance. Further, it
has been found that the fluorescent
substances becomes phosphorescent if
fixed by suitable methods, e.g. by fusion with
other substances etc. For example, many dyes
(which show fluorescence) when dissolved in
fused boric acid or glycerol and then cooled to
get rigid mass become phosphorescent.
FLUORESCENCE AND PHOSPHORESCENCE IN TERMS OF
EXCITATION OF ELECTRONS (JABLONSKI DIAGRAM)
• When energy in the form of light radiations falls on
certain molecules. some of the electrons present in
them absorb energy and jump to the outer orbitals.
The molecules are then said to be in the excited
state. Most of the excited species have very short life
times, usually less than 10 -6 second They may either
undergo a chemical change or return to the ground
state.
• The excitation of an electron occurs so rapidly (< 10-13
second) that it is believed that there is not enough
time for the orientation of the spin of the electron to
change, i.e., the one which was clockwise remains
clockwise and the other which is anti-clockwise
remains anti-clockwise after the excitation. Thus in
terms of electron spin, we can represent an excitation
as follows
• These states are usually expressed in terms of a
quantity called spin multiplicity which is equal to 2S +
1 where S is the total electron spin. For a molecule
containing all paired electrons S=0and so its spin
multiplicity = 1. The molecule is said to be in singlet
ground state. After absorbing photon, when the
molecule is in excited state with opposite spins, S= ½
-½= 0 so the spin multiplicity = 1 . The molecules is
then said to be in singlet excited state. When in the
excited state the electrons are present in different
orbitals but with spin in the same direction, S= ½ + ½
=1 and spin multiplicity= 2S + 1 =3. The molecule is
then said to be in triplet excited state.
•
• Depending upon the energy absorbed, electrons may
jump to any of the higher excited state and we have a
number of singlet excited states called first, second,
third singlet excited state represented by S1, S2, S3
etc. The singlet ground state is represented by S0. The
corresponding triplet excited states are called first,
second, third triplet excited states and are represented
by T1, T2, T3 etc. Any singlet excited state is found to
have higher energy than the corresponding triplet
state i.e. ES1 > ET1, Es2 > ET2 and so on.
•
The excited species can return to the
ground state by losing all of its excess energy by anv
one of the paths as shown in Fig., called Jablonski
diagram
• In all these paths, however the first step is the transitions from
higher excited singlet state to the lowest excited singlet state
S1. This is called internal conversion (IC). The energy is lost
only in the form of heat (shown by wavy line in the diagram)
due to collision with other molecules.As it does not result into
the emission of any radiation, it is therefore, called nonradiative or radiation less transition It is a very fast process
and occurs in less than about 10-11 second. Now from the first
excited singlet state, the molecules return to the ground state
by any one of the following paths.
•
Path I. The molecule may lose rest of the energy also in
the form of heat so that the complete path is non-radiative.
•
Path II. The molecule may release the energy in the form
of light or ultraviolet radiations in returning to the ground state
from the first excited singlet state (shown by straight line in the
diagram). Hence it is a radiative transition. This is called
fluorescence. It occurs for about 10-8 second after absorption
so that a substance fluoresces only in the presence of
absorbed radiation.
•
• Path III. Some energy may first be lost in transition from first
excited singlet state to the first excited triplet state i.e. between
states of different multiplicity i.e. S1 to T1(or similarlyS2 to T2. S3
to T3 etc., in the form of heat so that it is again a radiationless
transition. It is also called intersystem crossing (ISC). Such
transitions are normally forbidden but whenever they occur,
their rate is very slow. In fur transition from first excited triplet
state to the ground state, the rest of the energy may again be
lost in the form of heat due to molecular collisions. Thus the
complete path is non-radiative.
• Further, the energy is lost not only as heat but also in
undergoing a chemical reaction .As singlet excited states return
quickly to the ground state, there is no chance for a chemical
reaction However, if the molecules are in the triplet excited
states, they return to the ground state slowly and thus get
sufficient time to undergo a chemical reaction. Thus molecules
undergoing chemical reaction are those which are first in the
excited triplet state.
• Path IV. After intersystem crossing, the molecule may lose the
rest of the energy- in the form of light in going from the excited
triplet state to the ground state. This is called
phosphorescence. As triplet states have much longer life
times than singlet states, some as long as an hour,
phosphorescence persists even after the removal of absorbed
radiation. This may also be attributed to the fact that
the
transitions from excited triplet states to ground singlet state are
forbidden.
•
Note that fluorescence involves a radiative transition
between two states of the same multiplicity (usually two
singlets) whereas phosphorescence arises due to a radiative
transition between two states of different multiplicity (usually
triplet to singlet). It may be observed that the energy emitted
during fluorescence as well as phosphorescence is less than
the energy absorbed during excitation, as a part of the energy
is dissipated as heat in the internal conversion and intersystem
crossing. Hence the frequencies emitted during fluorescence
and phosphorescence are smaller or wavelengths are longer
than those of the light absorbed for excitation.
•
• The paths followed as explained above may be
compared with a two-floor building having
steps, ramps and windows. First step of all
paths is like a ball falling from second floor to
the first floor by bouncing on steps going from
second floor to first floor. If the ball now falls on
the ramp going from first floor to ground floor, it
is like path I. If the ball after coming to first floor
falls out of the window, it is like path II. If after
coming to first floor, the ball moves on some
steps and then falls in the ramp going from first
floor to ground floor, it is like path III. If the ball
after having come to first floor, moves on some
steps going from first floor to ground floor and
then jumps out of the window, it is like path IV.
PHOTOSENSITIZATION
• There are many substances which do not react directly when
exposed to light. However if another substance is added, the
photochemical reaction starts. The substance thus added itself
does not undergo any chemical change. It merely absorbs the
light energy and then passes it on to one of the reactants. Such
a substance which when added to a reaction mixture helps
to start the photochemical reaction but itself does not
under go any chemical change is called a photosensitizer
and the process is called photosensitization. Thus a
photosensitizer simply acts as a carrier of energy.
•
At the first instance it appears that a photosensitizer is
similar to a catalyst. However, this is not so as they act by
different mechanism.
•
Examples of Photosensitized Reactions. A few well
known examples of the photosensitized reactions are briefly
described below
• 1. Dissociation of hydrogen molecules in presence
of mercury vapour. Hydrogen molecules do not
dissociate when exposed to ultraviolet light (of 2537 A
emitted by the mercury vapour lamp). However, when
hydrogen gas is mixed with mercury vapour and then
exposed to ultraviolet light, hydrogen molecules
dissociate to give hydrogen atoms through the
following reactions :
• Hg + hv → Hg*
• Hg* + H2 → Hg + 2H
• where Hg* represents the activated mercury atom.
Thus in the above reaction, mercury acts a
photosensitizer.
•
The H-atom being highly reactive can easily
reduce metallic oxides, nitrous oxide, carbon
monoxide etc.
• In a similar manner, the dissociation of NH3, PH3 AsH3 and a
number of organic compounds is photosensitized by mercury.
Certain combination reactions e.g.,
• 2H2 + 02-------> 2H20 ; N2 + 3H2------2NH3; 3O2-----> 2O3 etc.
are also mercury photosenstized.
•
Two well known mercury photosensitized reactions involving
dissociation of H2 molecules into H atoms are briefly explained
below :
• (a) Combination between CO and H2 to form formaldehyde
and glyoxal. It is believed to take place through the following
steps :
• (i) Hg + hv → Hg* (Activation)
• (ii) Hg*+H2 → 2H + Hg (Dissociation)
• (iii) H + CO → HCO
• (iv) HCO + H2 → HCHO + H
• (v) 2HCO → HCHO + CO
• (vi) 2HCO → OHC—CHO (Glyoxal)
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