Photochemistry

Photochemistry is the study of the interaction of electromagnetic radiation with matter resulting into a physical change or into a chemical reaction .

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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.

Laws Governing Absorption Of Light

• Lambert’s Law: This law states that decrease in the intensity of monochromatic light with the thickness of the absorbing medium is proportional to the intensity of incident light.

-dI/dx α I

or -dI/dx=KI,which on integration changes to

I=I₀ e^-Kx

Where I ₀ = intensity of incident light.

I=intensity of transmitted light.

K= absorption co efficient.

• Beer’s Law :It states that decrease in the intensity of monochromatic light with the thickness of the solution is not only proportional to the intensity of the incident light but also to the concentration ‘c’ of the solution.

Mathematically, -dI/dx α Ic

or -dI/dx = Є Ic ,which on integration changes to I=I₀ e^{- ЄCX}

Where, Є = molar absorption coefficient or molar extinction coefficient.

Laws governing Photochemistry

• Grotthus-Draper Law:

Only the light which is absorbed by a molecule can be effective in producing photochemical changes in the molecule.

• Stark-Einstein’s Law ( Second Law of Photochemistry):

It states that for each photon of light absorbed by a chemical system, only one molecule is activated for a photochemical reaction. The energy absorbed by one mole of the reacting molecules is given by E=Nhv. This energy is called one einstein.

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Numerical value of Einstein

• In CGS Units

E=2.86/λ(cm) cal per mole

or

2.86X10⁵ / λ(A⁰) K cal per mole

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• In SI units

E=0.1197/λ(m) J mol ^-1

Or

11.97X10^-5/λ(m) KJ mol^-1

Interpretation Of Einstein’s Law

In terms of Quantum efficiency :

Quantum Efficieny ф=

No. of molecules reacting in a given time

No.of quantas of light absorbed in the

same time

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Experimentally,

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Ф =rate of chemical reaction

quanta absorbed per second.

Quantum Yield

• In the photolysis of Cl₂ and H₂, Φ_HCl can be as high as 1 million.

Cl₂ + hν → 2Cl

Cl + H₂ → HCl + H (exothermic)

H + Cl₂ → HCl + H

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• In the photolysis of Br₂ and H₂, Φ_HBr is very low i.e about 0.01

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Br₂ + hν → 2Br

Br+ H₂ → HBr+ H (endothermic)

H + Br₂ → HBr + Br

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Luminescence

• The glow produced in the body by methods other than action of heat i.e. the production of cold light is called Luminescence. It is of three types:
• 1. Chemiluminecence: The emission of lighjt in chemical reaction at ordinary temperature is called Chemiluminescence

e.g. The light emitted by glow-worms

Fluorescence: Certain substances when exposed to light or certain other radiations absorb the energy and then immediately start re-emitting the energy. Such substances are called fluorescent substances and the phenomenon is called fluorescence .

e.g Organic dyes such as eosin,fluorescein etc.

vapour of sodium,mercury,iodine etc.

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Phosphorescence: There are certain substances which continue to glow for some time even after the external light is cut off. Thus, phosphorescence is a slow fluorescence.

Fluorescence and phosphorescence in terms of excitation of electrons:

Singlet ground state So

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Singlet excited state S₁

(pair of electorns with

Opposite spins but each

in different orbital)

Triplet excited state T₁

(pair of electrons with

parallel spins in different

Orbitals)

The exicted species can return to the ground state by losing all of its excess energy by any one of the paths shown in jablonski diagram.

Jablonski Diagram

Allowed singlet states:

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Forbidden triplet states

due to spin conversion

The first step is the transition from higher excited singlet states to the lowest excited singlet state S₁.This is called internal conversion (IC).It is a non-radiative process and occurs in less than 10^-11 second .Now from S₁ the molecule return to ground state by any of the following paths.

Explanation of Jablonski Diagram

• Path I : The molecule may lose rest of the energy also in the form of heat so that the complete path is non-radiative.

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• Path II: Molecule releases energy in the form of light or uv radiation.This is called Fluorescence

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• Path III :Some energy may be lost in Tranfer from S₁ to T₁ in the form of heat.

It is called intersystem crossing (ISC). This path is non-radiative.

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• Path IV : After ISC,the molecule may lose energy in the form of light in going from the excited triplet state to the ground state.This is called phosphorescence.

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photosensitisation

• Photosensitized reactions:
• An electronically excited molecule can transfer its energy to a second species which then undergoes a photochemical process even though it was not itself directly excited.
• Mercury acting as a photosensitizer:

Hg+hv Hg*

Hg*+H₂ Hg+2H

• Chlorophyll acting as a photosensitizer

CO₂+H₂O+hv chlorophyll 1/6(C₆H₁₂O₆)+O₂

Quenching of Fluorescence –

STERN-VOLMER EQUATION

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If the excited molecules are deactivated and the fluorescence stops,the phenomenon is called ‘Quenching’.

• When the activated molecules undergo a change from a singlet excited state to triplet excited state.This is called ‘internal quenching’.
• When the activated molecules collide with the other molecules/quenchers which are the externally added species and transfer their energy to those molecules.This is called ‘external quenching’.

A+hv A* (Activation)

A* k₁ A+hv (Flourescence)

A* k₂ A (Internal quenching)

A*+Q k₃ A+Q’ (External quenching)

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Intensity of the light absorbed:

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Ia=k₁[A*]+k₂[A*]+k₃[A*][Q]

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If I_f represents the intensity of fluorescence,

Ф_f or ф_q=I_f/Ia=k₁[A*]/k₁[A*]+k₂[A*]+k₃[A*][Q]

=K1/K1 +K2+K3[Q]

In the absence of the quencher,the quantum yield

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ф₀=k₁/k₁+k₂

ф₀/ф_q=k₁+k₂+k₃[Q]/k₁+k₂=1+k₃[Q]/k₁+k₂

Put 1/k₂+k₂=ѓ

φ₀/φ_Q=1+k₃ Ѓ[Q]

or

φ₀/φ_Q=1+k_sv[Q]

This is know as Stren-volmer equation.

Photochemical Equilibrium

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light

A B

Thermal

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Rate of forward reaction α I_abs=k₁I_abs

Rate of the backward reaction α[B]=k₂[B]

At Equiliubrium,

Rate of forward reaction=Rate of backward reaction

K₁ I_abs=k₂[B]

or

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K₂/k₁=I_abs/[B]

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or

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K=I _abs/[B]

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Where K=K₂/k₁ is the photochemical equlibrium constant