Basis of photochemistry

Photochemistry
ELECTROMAGNETIC SPECTRUM
LAW GOVERNING ABSORPTION OF LIGHT
LAW OF PHOTOCHEMISTRY
Grotthurs-Drapper law.
Einstein Stark law of photochemical equivalence
ELECTRONIC TRANSITIONS
Jablonski Diagram
QUANTUM YIELD
Use Of Photochemistry
Chemistry of vision
Photosynthesis in plant
Formation of Vitamin D
Fluorescent dyes in traffic
Photodynamic therapy

1. Basis Of Photochemistry PRESENTED BY : KESHAV KUMAR SINGH Reg. No.:- Y19266023 Deptt. of Chemistry DHSGU Sagar (M.P.)

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 up to meters.

5. 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.

6. 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 l or dI/ dx = kl

7. 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 l × c or dI/ dx = lcɛ

8. Combined Lambert-Beer’s Law is given as: log I / I = ɛcl = A Where I = 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.

9. 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.

10. 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)

11. The energy absorbed by one mole of the reacting molecule is given by E = N A hν …..(1) Where N A is a Avogadro’s number Also ν = c/λ so (1) becomes E = N A hc/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.

12. ELECTRONIC TRANSITIONS 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. σ → σ *   Transition n → σ *   Transition π → π* Transition n→ π * Transition

13. Jablonski Diagram

14. 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 10 -11 second. Now from S 1 the molecule return to ground state by any 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 .

15. 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 .

16. QUANTUM YIELD It is the number of molecule reacting per quantum of light absorbed. It is denoted by ɸ (Phi). Mathematically: The quantum yield of the product formation is given by The quantum yield may be as high 106 or as low as for several photochemical reactions.

17. 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. Use Of Photochemistry Chemistry of vision Photosynthesis in plant Formation of Vitamin D Fluorescent dyes in traffic Photodynamic therapy