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

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=I 0 e -Kx Where I 0 = intensity of incident light. I=intensity of transmitted light. K= absorption co efficient.

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.

Numerical value of Einstein In CGS Units E=2.86/λ(cm) cal per mole or 2.86X10 5 / λ(A 0 ) K cal per mole  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 Experimentally, Ф =rate of chemical reaction quanta absorbed per second.

Quantum Yield In the photolysis of Cl 2 and H 2,  HCl can be as high as 1 million. Cl 2 + h  2Cl Cl + H 2  HCl + H (exothermic) H + Cl 2  HCl + H In the photolysis of Br 2 and H 2,  HBr is very low i.e about 0.01 Br 2 + h  2Br Br+ H 2  HBr+ H (endothermic) H + Br 2  HBr + Br

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. 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 Singlet excited state S 1 (pair of electorns with Opposite spins but each in different orbital) Triplet excited state T 1 (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: Forbidden triplet states due to spin conversion

The first step is the transition from higher excited singlet states to the lowest excited singlet state S 1.This is called internal conversion (IC).It is a non- radiative process and occurs in less than second.Now from S 1 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.

 Path II: Molecule releases energy in the form of light or uv radiation.This is called Fluorescence  Path III :Some energy may be lost in Tranfer from S 1 to T 1 in the form of heat. It is called intersystem crossing (ISC). This path is non- radiative.  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.

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*+H 2 Hg+2H  Chlorophyll acting as a photosensitizer CO 2 +H 2 O+hv chlorophyll 1/6(C 6 H 12 O 6 )+O 2

STERN-VOLMER EQUATION 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’. Quenching of Fluorescence –

A+hv A* (Activation) A* k 1 A+hv (Flourescence) A* k 2 A (Internal quenching) A*+Q k 3 A+Q’ (External quenching) Intensity of the light absorbed: I a =k 1 [A*]+k 2 [A*]+k 3 [A*][Q] If I f represents the intensity of fluorescence, Ф f or ф q =I f /I a =k 1 [A*]/k 1 [A*]+k 2 [A*]+k 3 [A*][Q] =K1/K1 +K2+K3[Q]

In the absence of the quencher,the quantum yield ф 0 =k 1 /k 1 +k 2 ф 0 /ф q =k 1 +k 2 +k 3 [Q]/k 1 +k 2 =1+k 3 [Q]/k 1 +k 2 Put 1/k 2 +k 2 =ѓ φ 0 /φ Q =1+k 3 Ѓ[Q] or φ 0 /φ Q =1+k sv [Q] This is know as Stren-volmer equation.

light A B Thermal Rate of forward reaction α I abs =k 1 I abs Rate of the backward reaction α[B]=k 2 [B] At Equiliubrium, Rate of forward reaction=Rate of backward reaction Photochemical Equilibrium

K 1 I abs =k 2 [B] or K 2 /k 1 =I abs /[B] or K=I abs /[B] Where K=K 2 /k 1 is the photochemical equlibrium constant