Dinkars presentation on potochemistry.
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This document summarizes a seminar on photochemistry presented by Mr. Dinkar B. Kamkhede. The seminar covered topics including the definition of photochemistry, laws of photochemistry, mechanisms of light absorption, electronic transitions, photosensitization, and the Jablonski diagram. It discussed how photochemical reactions are initiated by the absorption of light energy and explained concepts such as quantum yield. The seminar provided an overview of the key concepts and processes in photochemistry.
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Basis of photochemistry
Photochemistry
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LAW GOVERNING ABSORPTION OF LIGHT
LAW OF PHOTOCHEMISTRY
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PHOTOCHEMISTRY BASIC PRINCIPLE AND JABLONSKI DIAGRAM
Photochemistry is the branch of chemistry in which study of chemical reactions take place by
the absorption of electromagnetic radiation or by molecules absorb light radiation
(electromagnetic radiation) particularly the visible (wavelength from (400-750) and ultra violet region (wavelength from 100-400nm ), the molecules generally get activated due to
electronic excitation. When electromagnetic radiation is absorb in the ultraviolet/visible
region the molecules get excited to higher electronic state. This involves the promotion of an
electron from bonding molecular orbital to antibonding molecular orbital. According to the quantum theory, both matter and light are quantised, and only certain
specific energies of light are absorbed by specific organic molecule for its excitation. The
absorption or emission of light occurs by the transfer of energy as photons. All photochemical and photo physical processes are initiated by the absorption of a photon of visible or ultraviolet radiation leading to the formation of an electronically - excited state. The molar absorptivity (formerly called the extinction coefficient) of a compound constant
that is characteristic of the compound at a particular wavelength. The Jablonski diagram is a pictorial illustrated of different energy states which are absorbed by molecules. This
partial energy diagram represents the energy of a photo luminescent molecule in its different energy states.The life time of singlet excited state S1 is long hence in this state has done many physical and chemical processes. Molecules returns to its ground state, S0 from excited singlet S1,/ S2 state by release energy as heat, but this is generally quite slow because the amount of energy is large between S0 and S1. This process is called internal conversion. When molecules return to its ground state S0 from excited state S1,/ S2 by giving off
energy in the light form within 10-9 seconds. This process is known as Fluorescence.
This pathway is not very common because it is relatively slow. For smaller, diatomic
and rigid molecules (mainly aromatic compounds) show fluorescence. This is because
emitted fluorescent light is of lower energy than absorbance light. Most molecules in the S1 state may drop to triplet state (T1) (S1→T1). This is
energetically slow process. However, if the singlet state S1 is long lived, the S1 →
T1 conversion occurs by a process called intersystem crossing. It is important
phenomenon in photochemistry. For every excited singlet state there exist
corresponding triplet states. Since transition from ground state singlet (S0) to triplet
state (T1) is forbidden, intersystem crossing is the main source of excited triplet
state. This is one way of populating the triplet state. The efficiency in intersystem
crossing depends on the S1 → T1 energy gap.
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- 1. SEMINAR ON PHOTOCHEMISTRY Presented By:- Mr.Dinkar B.Kamkhede MSc.Chemistry,IIYear 2013-2014 Vidyabharati Mahavidyalay, Amravati.
- 2. Introduction Photochemistry is concerned with the changes in chemical and physical behaviour of molecules following absorption of one or (more) photons. Primarily consider absorption of visible/UV although IR absorption may also change chemical behaviour Mainly concerned with electronic excitation, usually accompanied by some vibrational excitation And (rotational in gas phase ) excitation. Contents- Introduction Phoyochemistry Intracion of Radition with matter Electronic Transition Laws of Photochemistry Mechanisim of Light Absorption Photosensitisation Joblonski Digram Modes of Dissipation of Energy Energy Transfer
- 3. Photochemistry Chemical reactions- 1 .Thermal Reaction-The Reaction proceed with the absorption of heat energy. Action of light → chemical change (light induced reactions) 2 .Photochemical Reaction-The Reaction takes place by the absorsption of light energy. The Chemical reaction that are initiated or affected by Light called photochemical reaction. Chemical reaction → light emission (chemiluminescence)
- 4. Intraction of Radiations with Matter Light: electromagnetic field vibration spreading in quanta (photons) Photon: the smallest amount of light carrying energy Quantise- transition means for a particular energy molecule absorbed specific amount of energy.which lead to chemical change in a molecule due to absorption of EMR. Energy of photons (A. Einstein)- E = c h h= h = Planck’s constant (6.6 · 10-34 Js) c = speed of light (3 · 108 ms-1) l = wavelength n = frequency Einstein’s Equivalency Principle- One particle of a chemical substance can absorb only one photon from a light beam: DE = h
- 5. Types of Excitation Organic Compound σ → σ* Alkane,Which has only 6 -bon n → σ* Alcohol,Amine,Ether, Thioether,etc π → π* Alkene compound,etc n → π* Corbonyl Compound,etc Electronic Transition`s
- 6. Photochemical Process given two Laws of Photochemistry 1. (Grotthus, Draper) - 1st Law of Photochemistry 2. (Stark, Einstein) - 2nd Law of Photochemistry 3. Q uantum yield or Quantum Efficiency 4. if Law is correct then quantum yield should be unitly.This law ever is very rare. - number of molecules undergoing the process number of quanta absorbed=
- 7. Mechanisms of Light Absorption Excitation: X2 h *X2 A bonding electron is lifted to a higher energy level (higher orbital) INTERACTION OF LIGHT AND MATERIALS: a) X2* → X2 + M* (excess energy transferred to the surrounding) b) X2* → X2 + hn (fluorescence or phosphorescence) c) X2* + Y → chemical reaction (excess energy supplies the activation energy of the reaction)
- 8. When any transition attain Triplet state it return back to GS , i.s Energy gap between T1-So is constant.f or a perticular type of transition. during Phosphorasence emitted in energy may be absorbed by other molecule o produce its own curresponding T.S. called Photosensitization Photosensitization
- 9. Jablonski Diagram It explain discription of molecule present in different Energy level after absorption of EMR.
- 10. Modes of Dissipation of Energy (S0) (S1) 10-8s (S2) 10-11s (T1) 10-3s-1s S2 : The higher vibrational level of the excited singlet state S1 IC: Internal conversion; RD: Radiative deactivation F: Fluorescence (spin consevation); ISC: Inter system crossing P: Phosphorescence (Spin inversion). h IC RD F ISC P RD (Jablonski diagram) Deactivation no radiative IC ISC (Spin inversion) radiative F P S1 T1 + + + photosensitization
- 11. Energy transfer through photosensitization D 1D h 1D 3D ISC A + 3D D + 3A 3A Products D = Donor A = Acceptor 1 = Singlet 3 = Triplet S0 S1 74 Kcal .mole-1 69 Kcal/mole T1 ISC 120 Kcal/mole S0 T1 S1 60 Kcal/mole Energy transfer Benzophenone Butadiene Ph2CO h 1[Ph2CO] ISC 3[Ph2CO] + Ph2CO 3 Dimeric products
- 13. Criteria of an ideal sensitizer • It must be excited by the irradiation to be used, small singlet triplet splitting. High ISC yield. • It must be present in sufficient concentration to absorb more strongly than the other reactants under the condition. • It must be able to transfer energy to the desired reactant, low chemical reactivity in Triplet state. Internal conversion • Internal conversion nearly always involves change of orbital configuration. • Nuclear kinetic energy operator is totally symmetric, suggesting IC is formally forbidden. • However separation of Franck Condon factor not strictly valid because Hamiltonian depends on nuclear co-ordinates.