1. PHOTOCHEMISTRY AJAY SHARMA S/O Sh. Ashwani Kumar
V.P.O- Bhaddi, Teh. – Balachour
Distt- S.B.S Nagar, Punjab, India
PIN:
Mobile:

2. INTRODUCTION
Photochemistry is concerned with the absorption, excitation and emission of photons by atoms, atomic ions, molecules and molecular ions etc.
It deals with the study of interaction of radiation with matter resulting into physical changes or into a chemical reaction.
The term radiation includes all type of electromagnetic waves from very low frequency microwave to high frequency X-ray and γ-rays.
The radiation of photochemical importance are visible and UV radiation.

3. Photochemistry start with the absorption of light radiation by atom or molecules which brings about the excitation of atoms or molecules followed by physical or chemical changes.
In photochemical reactions two main processes are Photo-physical and Photochemical processes.
Photo-physical processes include fluorescence, phosphorescence and photoelectric effect where as photochemical processes include chemical reactions that occur in the presence of light such as photosynthesis of carbohydrates in plants, decomposition of ozone and formation of vitamin-D in human body.

4. ELECTROMAGNETIC SPECTRUM
The arrangement of all types of electromagnetic radiation in the order of increasing wavelength or decreasing frequency is called electromagnetic spectrum.
In electromagnetic spectrum gamma rays and X-rays have high energy but lower wavelength where as microwaves and radio waves have lower energy but higher wavelength.
In electromagnetic spectrum X-rays region extends from to 100nm, UV 100 to 400nm, Visible 400 to 800nm, IR 0.8 to 200μm after this region of microwaves and radio waves start which have wavelength upto meters.

6. The photons of electromagnetic radiation of different wavelength interact with chemical species in different ways.
In the gas phase molecules are made to rotate by microwaves.
Infrared radiation cause molecular bond to stretch and vibrate that’s why it often called vibrational spectroscopy.
Visible radiation induces low energy electronic transition in atoms and molecules
UV radiation causes high energy electronic transition in molecule. The resulting excited states may relax by bond breakage.
X-rays excite and eject inner shell electrons. This causes widespread ionization and bond fragmentation.
Gamma rays causes high energy transition in atomic nuclei.

7. PHOTOCHEMICAL ENERGY
Photochemical energy of electromagnetic radiation is given as: E = hν ν = c/λ Therefore E = hc/ λ Where ν = frequency of electromagnetic radiation. λ = wavelength of electromagnetic radiation. C = velocity of light h = Planck constant

8. LAW GOVERNING ABSORPTION OF LIGHT
The fraction of light absorbed (I/I0) is given by the Lambert’s-Beer’s law: Lambert’s law: 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 is proportional to the intensity of the incident light. Mathematically: dI/dx α I or dI/dx = kI

9. Beer’s law: When a monochromatic light is passed through a solution, the decrease in the intensity of light with thickness of the solution is directly proportional to the intensity of the incident light and the concentration of the solution.
Mathematically :
dI/dx α I × c
or dI/dx = Icɛ

10. Combined Lambert-Beer’s Law is given as: log I0 / I = ɛcl = A Where I0 = Intensity of the incident light. I = Intensity of the transmitted light. c = Concentration of the solution in moles/litre. l = Path length of the sample usually 1cm. ɛ = Molar absorptivity or molar extinction coefficient. A = Absorbance or optical density.

11. Limitations:
The Beer’s – Lambert Law is obeyed when a single species gives the observed absorption. But law is not obeyed when:
Different forms of the absorbing molecules are in equilibrium. For example keto-enol tautomerism.
Solute and solvent form complexes due to association.
There is a thermal equilibrium between ground electronic state and a low –laying excited state.
Fluorescent compounds are present.

12. MOLAR EXTINCTION COEFFICIENT
Molar extinction coefficient is a property of the molecule undergoing an electronic transition.
Mathematically :
ɛ = A /cl
Its magnitude depends upon the probability of transition and the size of Chromophore.
The typical values of molar extinction coefficient vary from
Values above 104 are termed high intensity absorption while values below 103 are called low intensity absorption.

13. LAW OF PHOTOCHEMISTRY
The photochemical process are governed by the following laws:
Grotthurs-Drapper law.
Einstein-Stark law of photochemical equivalence
Grotthurs-Drapper law:
When light falls on a body, a part of it is reflected, apart of it is transmitted and rest of it is absorbed. It is only the absorbed light which is effective in bringing about a chemical reaction.
The law is purely qualitative and does not gives any relationship between the amount of light absorbed by a molecule and the molecule which have reacted.

14. Einstein-Stark law:
According to this law each quantum of light absorbed by a molecule, activate only one molecule in the primary step of photochemical process or briefly one molecule one quantum.
AB hν = AB*
One One Activated molecule
molecule photon (excited molecule)

15. The energy absorbed by one mole of the reacting molecule is given by
E = NAhν …..(1)
Where NA is a Avogadro’s number
Also ν = c/λ so (1) becomes
E = NAhc/λ ….(2)
Where c velocity of light, λ is the wave length of the absorbed light and h is Plank’s constant.
The energy possessed by one mole of photon is called one einstein.

16. QUANTUM YIELD
It is the number of molecule reacting per quantum of light absorbed. It is denoted by ɸ (Phi). Mathematically:
ɸ =
The quantum yield of the product formation is given by

17. Low quantum yield is due to
The quantum yield may be as high 106 or as low as for several photochemical reactions.
Low quantum yield is due to
The excited molecule may get deactivated before they form products.
Collision of the excited molecules with non-excited molecules may cause the former to lose their energy.
The primary photochemical process may get reversed.
The dissociated fragments may recombine to form the original molecule.

18. High quantum yield is due to
The primary process of absorption of radiation produces excited free radicals. They undergo secondary processes which again produces excited free radicals. This process is continues unless it is controlled. Thus by absorbing only one quantum of radiation, several reactant molecules undergo chemical reaction. Hence ɸ will be greater than unity.
Free radical gives chain reaction which increases quantum yield of the reaction.

19. ELECTRONIC TRANSITIONS
Only light of discrete frequencies will cause a transition to occur because energy difference between electronic energy levels is quantized.
E = hv (h= Planck’s constant, v = frequency)
Electron in the ground state move to an excited state if energy is supplied, in photochemical reaction this energy is in the form of light.
Functional group containing multiple bond that absorbs electromagnetic radiations are called chromophore.

20. GROUND STATE AND EXCITED STATE

21. SINGLET AND TRIPLET STATES
All electrons in the ground state are paired having an opposite spin.
If one of the electrons in pair is promoted to orbital of higher energy, the promoted electron may posses either a parallel or opposite spin.
If a molecule contains two unpaired electrons of same spin the state is called a triplet state.
If a molecule contains two unpaired electrons of opposite spins the state is called a singlet state.

22. SINGLET AND TRIPLET STATES

23. For every singlet state there is a corresponding triplet state .
Triplet state is of lower energy than the corresponding singlet state
The electron from the ground state is either promoted to an excited triplet state or excited singlet state, just a different amount of energy is required.
These transitions may be allowed or forbidden.

24. SELECTION RULES
Transitions in which an electron changes its spin are called forbidden transitions.
Transitions from singlet to triplet and triplet to singlet are forbidden and singlet to singlet and triplet to triplet are allowed transitions.
Forbidden transitions occur slowly with low intensity.
The different rate of photochemical reactions is due to difference in rate of allowed and a forbidden transitions.

25. TYPES OF EXCITATION
When an electron in a molecule is promoted it goes from the highest occupied molecular orbital (HOMO) into the lowest unoccupied molecular orbital (LUMO). This promotion of electron is known as electronic excitation.
The four possible electronic excitations are (in order of decreasing energy):
1. s  s*
2. n  s*
3. p  p*
4. n  p*

26. ELECTRONIC TRANSITIONS

27. Photochemical transition

28. Effect of conjugation on electronic transitions
Increased conjugation shift the intensity of absorption of chromophore towards longer wavelength.
As the conjugation increased the HOMO and LUMO gap decreases hence the wavelength of absorption increases.

29. With enough conjugated double bonds, intensity of absorption shifted to the visible region.
For example b-carotene from carrots (orange) with eleven conjugated double bond absorb at 455 nm and lycopene from tomatoes (red) also with eleven conjugated double absorb at 474 nm.

30. LUMINESCENCE
The production of cold light is called luminescence. The body emitting the cold light is called luminescent.
Luminescence is of three type:
Chemiluminescence
Fluorescence
Phosphorescence

31. CHEMILUMINESCENCE Examples:
The emission of light in a chemical reaction at ordinary temperatures is called Chemiluminescence
Examples:
The light emitted by the glow worms due to oxidation of protein luciferin by atmospheric oxygen in the presence of enzyme luciferase.
Oxidation of yellow phosphorus in air to give P2O5 at ordinary temperature.

32. Luciferin (fireflies): Phosphorescence occur in luciferin due to degradation of a 4-membered ring which is formed by the reaction of luciferin with oxygen in the presence of ATP and enzyme luciferase to give oxyluciferin through a carbanion intermediate.

33. FLUORESCENCE
Certain substances which when exposed to light absorb the energy and then immediately start re-emitting the energy. Such substances are called fluorescent and the phenomenon is called fluorescence.
It is an immediate form of radiative decay occur within sec.
S1  S hv (Fluorescence)
The wave length of the emitted light is greater than the absorbed light.
Examples are CaF2, vapour of sodium, mercury and iodine, inorganic compound like Uranyl sulphate (UO2SO4).

34. FLUORESCENCE
FLUORESCENCE

35. PHOSPHORESCENCE It is a slow fluorescence.
Certain substances which continue to glow for some time even after the external light cut off. Such substances are called phosphorescent and the phenomenon is called phosphorescence.
In phosphorescence the intersystem crossings is the key step, that is S1 T1 .
The triplet state acts as a slowly leaking reservoir.
Example are zinc sulphide, sulphides of alkaline earth metals and production of light in fireflies (luciferin).

36. PHOSPHORESCENCE

37. The Fate of Excited Molecule: Jablonski Diagram :

38. QUENCHING OF FLUORESCENCE
When the excited molecule are deactivated and fluorescence stop the phenomenon is called quenching.
The substances which are responsible for stopping the fluorescence are called quencher.
Quenching may occur in two ways
When a activated molecule undergo a change from a singlet excited state to the triplet excited state. This is called internal quenching.
When the activated molecules collide with the other molecules which are externally added species and transfer their energy to those molecules. This is called external quenching.

39. Various steps involve during the process of quenching
A + hv → A* (Activation)
A* → A + hv (fluorescence)
A* → A (Internal quenching)
A* + Q → A + Q* (External quenching)

40. PHOTOSENSITIZATION
A substance which when added to a reaction mixture helps to start the photochemical reaction but itself does not undergo any chemical change is called photosensitizer and the process is called photosensitization.
The photosensitizer simply acts as a carrier of energy
D hv → D*
D* + A → D + A*
D = Donor A = Acceptor

41. PROPERTIES OF PHOTOSENSITIZER
It must be excited by the radiation to be used.
It must absorb more strongly than the other reactants under the condition of experiment.
The energy of the triplet state of sensitizer must be greater than that of the reactant.
It must be able to transfer energy to the desired reactant.
The sensitizer should have high ISC and absorb at lower wavelength.

42. EXAMPLES OF PHOTOSENSITIZATION
Decomposition of Ozone in the presence of Chlorine as photosensitizer:
a) Cl2 + hv → 2Cl b) Cl + O3 → ClO3*
c) ClO3* + O3 → ClO O d) ClO2 + O3 → ClO3 + O2
Decomposition of Diazomethane (CH2N2) in the presence of Benzophenone as photosensitizer:
a) Bz + hv → Bz* b) Bz* + CH2N2 → Bz + CH2N2*
c) CH2N2* → CH2 + N2

43. Isomerization of But-2-ene from cis to trans in the presence of SO2 as photosensitizer:
a) SO2 + hv → SO2*
b) SO2* cis-But-2-ene → SO cis-But 2-ene*
c) cis-But-2-ene* → trans-But-2-ene* → trans-But-2-ene
Decomposition of oxalic acid in the presence of uranyl sulphate. This reaction takes place in solution.
a) UO hv → UO22+*
b) UO22+* + (COOH)2 → UO CO2 + CO + H2O

44. PHOTO-INHIBITORS
Certain substances which when present reduce the quantum yield of some photochemical reactions are called photo-inhibitors.
These substances react with the chain propagating atoms or radicals resulting into the chain termination.
Example are oxygen, nitric oxide, sulphur dioxide, propylene and cyanides.

45. EXAMPLE OF PHOTO-INHIBITOR
Presence of traces of oxygen in the photosynthesis of HCl acts as photo-inhibitor. It reduces the quantum yield of the reaction.
In the photosynthesis of HCl presence of oxygen terminate the chain propagating step of the reaction thus reduces the quantum yield.
a) 2H + O2 → H2O2
b) Cl O HCl → HO2 + Cl2
c) HO2 + H → H2O2

46. PHOTOCHEMICAL EQULIBRIUM
In a photochemical reaction a state may reach when the rate of forward reaction may become equal to the rate of backward reaction. This stage is known as photostationary state.
At this stage absorbance of light produces no further chemical change.
This situation is represented by either of the following two ways: or

47. EXAMPLES OF PHOTOSTATIONARY STATE
Photochemical decomposition of nitrogen dioxide
2NO NO + O2
Dimerization of anthracene
2C14H C28H20
Photochemical decomposition of sulfur trioxide
2SO SO2 + O2

48. Photochemical reaction takes place in two steps
1. Primary process: This involves the activation of certain molecules by absorption of light or the dissociation to produce active atoms or free radical i.e.
AB hv → AB*
Activated molecule
AB hv → A B*
Active or excited atom
2. Secondary process: The activated molecules of primary process reacts with other molecules in different ways or the deactivation of the activated molecules occur.

49. MULTIPLE REACTION PATHWAYS FOR ELECTRONICALLY EXCITED SPECIES

50. DISSOCIATION Photolysis of acetone: The reaction may be represented as
CH3COCH3 + hv → C2H CO
Acetone Ethane Carbon
monoxide
Along with these product small amount of methane and appreciable amounts of diacetyl (CH3CO)2 are also formed.
Mechanism
1. Primary process:
CH3COCH3 + hv → .CH CH3CO.
Acetone Methyl Acetyl
free radical free radical

51. 2. Secondary process: A number of secondary reactions are possible.
a) CH3CO. → .CH CO
b) .CH CH3CO. → C2H CO
c) CH3CO. + CH3CO. → (CH3CO)2
Degradation of ozone: Ozone layer has been thinning gradually. The thinning of ozone layer is due to the presence of chlorofluorohydrocarbons like CFCl3 and CF2Cl2 in the atmosphere.

52. These chemicals affect ozone concentration
a) CFCl hv → .CFCl Cl
Chlorine atom being highly reactive reacts with ozone (O3)
b) .Cl O3 → .ClO O2
The monoxide of chlorine further reacts with another molecule of O3
c) .ClO O3 → .Cl O2
The chlorine atom so obtained reacts with another ozone molecule. Hence, steps (b) and (c) are repeated again and again and, leads to the depletion of concentration of ozone.

53. NORRISH FRAGMENTATION
Norrish Type 1 fragmentation hυ
Norrish Type 2 fragmentation hυ

54. Norrish Type Reaction: Saturated carbonyl compounds undergo photoinduced decarbonylation in the gas phase called Norrish Type-1 process. In primary photoprocess cleavage of the carbonyl carbon and alpha carbon bond occur to give an acyl and an alkyl radicals. In secondary process an acyl and an alkyl radicals undergo numerous secondary reactions to give different products.

55. REARRANGEMENT
Photo induced rearrangements of stilbene
The cis-trans isomerisation of stilbene occurs through rotation around the double bond in the presence of light.

57. ABSTRACTION
Intramolecular Hydrogen Abstraction
Carbonyl compounds with a hydrogen atom attached to the fourth carbon atom undergo 1-5 intramolecular hydrogen transfer to give diradical. The resulting diradical either form cycloalkanol or undergo b C-C bond fission to give an alkene and enol. The enol is thermodynamically unfavourable and converts to a ketone.

58. Photo-reduction: The reaction of benzophenone in the triplet excited state with isopropanol to give diphenylketyl radical and dimethylketyl radical is an example of photo-reduction. The dimethylketyl radical produced transfers a hydrogen atom to benzophenone in the ground state to produce another diphenylketyl radical. It is interesting that only one photon is needed to convert two molecules of the reactant to product.

59. SUBSTITUTION
Photo-substitution reactions are characteristics of substituted aromatic compounds. For example the photoreaction of m-nitroanisole with cyanide ion . The mechanism involves a complex of the aromatic molecule in the triplet state with the nucleophile.

60. PHOTOSYNTHESIS
Photosynthesis of HCl from H2 and Cl2
H2 reacts Cl2 in the presence of light to give HCl the process is called photosynthesis.
H Cl hv → 2HCl
The quantum yield of this reaction is very high i.e. 104 to 106. This is due to the chain propagating reactions in secondary process.

61. i) Chain propagating steps
1. Primary process: Chain initiating step
Cl hv → 2Cl
Secondary process:
i) Chain propagating steps
a) CI + H2 → HCl + H + heat
b) H + Cl2 → HCl + Cl
ii) Chain termination step
c) CI + Cl → Cl2
In this reaction chain propagating steps repeat over and again till whole of H2 and Cl2 have reacted to form HCl due to this region the quantum yield of this reaction is high.

62. ADDITION REACTION
Photocycloaddition reactions: An addition reaction between two unsaturated compounds leading to the formation of cyclic product in the presence of light are called photocycloadditon reactions. For example (2+2) and (2+4) photocycloaddition reactions.
(2+2) Photocycloaddition:

63. (2+4) Photocycloaddition:
1-4 Photoaddition of Benzene:

64. PHOTO-OXIDATION
Photo-0xidation: The addition of singlet oxygen generated photochemically to double bonds is known as photo oxidation. For example addition of oxygen to anthracene in the presence of light.

65. CONCLUSION
Photochemical reactions occur only in the presence of electromagnetic radiation.
Photochemical reactions always occur in their excited state.
Photochemical reaction occur under mild condition of reagent, temperature and pressure.
Photochemical reactions are very clean reactions as very less or no side products are formed.

66. THANKS